Obviously people have found pragmatically you can get away without an expensive DFB
laser; that crude diodes can be SLM; it opens up the interesting qn of just why it
seems modern diodes are tending to go SLM spontaneously, & how stable the output
wavelength is when they do go SLM (order nm/degree from memory?)

(Прислал: Bret Cannon (bdcannon@owt.com).)

There are two temperature tuning rates for a diode laser, one is the tuning
of a given longitudinal mode with temperature and the other is the tuning
over larger temperature changes where the lasing mode hope from longitudinal
mode to longitudinal mode to be close to the peak of the gain curve. The
average tuning rate for this later rate is typically 0.3 nm/°C while for
small enough temperature changes the tuning of longitudinal mode is much
smaller. For a temperature stability of 1 mK a diode laser frequency is
stable to better than 0.001 cm-1, perhaps even a good as 0.0001 cm-1 as
determined by tuning onto a Doppler-free atomic transition. Thus at 780 nm
the temperature tuning of a longitudinal mode is less than 0.06 nm/°C. With
a temperature tuning of less than 1 cm-1/C, a temperature stability of 0.1 °C
during an exposure would give a coherence length longer than 10 cm.

Unless there is external optical feedback or a very sophisticated electronic
feedback there is no way that a 780 nm CD laser would have a linewidth of
10s of kHz. With a sufficiently low noise current supply (less than 1
microamp RMS in a 1 MHz bandwidth) and temperature stabilization to about 1
mK, the intrinsic linewidth of diode lasers can be measured and they are
proportional to the inverse of the output power. Linewidths of about 50 MHz
for a 3 mW laser and 5 MHz for a 30 mW laser are typical. These linewidths
are 5 to 50 times the Shawlow-Townes linewidth for these lasers and results
from the coupling of the refractive index and the population inversion.
Moradian (sp?) who was at MIT at the time published experimental
measurements in the late 1970s and early 1980s. Henry published an analysis
of this line broadening mechanism but I don't remember exactly when.

The linewidth decreases with the square of the cavity length and with
external cavities a few cm long people have achieved linewidths of less than
1 kHz. An example of this is work by Leo Holberg and colleagues at NIST in
Boulder for an optical clock based on an inter-combination line in optically
cooled and trapped atomic calcium.

Here are some of the things that manufacturers use to rate and promote
both red and green laser pointers:

Wavelength: This may be specified but don't trust it too much.
Usually, lower is better since visibility is a strong function of wavelength.
However, the variability can be significant - a pointer spec'd at 640 nm
could indeed be closer to 650 nm and there's almost a 2:1 ratio of relative
brightness between these extremes.

Output power: Power ratings are often made deliberately confusing
like "<5 mW", which could mean almost anything! Even among identical
models, there can be significant variation, especially for green DPSS laser
pointers.

Visibility: Manufacturers will often compare visibility to the
oldest 670 nm (or longer) wavelength. All but the cheapest pointers today
will be somewhere in the 640 to 650 nm range. Between the shortest available
wavelength of 635 nm and 670 nm is a ratio of relative brightness of about
7:1. Or, they'll compare a 1 mW 670 nm pointer to a 5 mW 640 nm unit and
claim "35 times brighter". Or for a green 5 mW pointer, "135 times
brighter". :) See the section: Equivalent
Brightness Ratings and Laser Pointer Visibility.

Distance/range: By itself, this is basically a totally useless
number. Do they mean on a moonless night under smog-free conditions? Light
doesn't travel a specific distance and stop, or suddenly become too dim to
be seen. It's only possible value would be in comparing various models from
the same manufacturer/supplier and even there, they probably just associate
a particular value with the output power and/or wavelength.

Beam shape/quality: Without significant effort, the output of a red
diode laser pointer is not a nice round spot. More expensive pointers may
have the necessary optics to do a decent job of beam shaping but most do not.
Sometimes, the beam shape will be shown in the catalog or Web site listing,
probably to convince you to upgrade to the model with the rounder spot.
Green DPSS laser pointers may claim a TEM00 beam and some come quite close
to perfection, though there will often tend to be a certain amount of
ghosting and extraneous little spots associated with it. For pointing at
normal viewing distances, unless the beam shape is absolutely terrible, it
doesn't really matter much.

CW or pulsed: As far as I know, all red laser pointers produce a
continuous (CW) beam. However, due to the way green DPSS lasers work, there
are significant advantages in terms of efficiency and thus battery to use a
pulsed system and many do. As a practical matter, it doesn't much matter
to the user unless the pointer is moved rapidly in which case the pulsed beam
will show up as discrete spots rather than a continuous line. (Note that
red pointers could also benefit from pulsing but it would require a higher
power more expensive laser diode and more complex driver, and the improvement
in battery life - which is already very adequate - would only be modest.)

APC or ACC driver: Most red pointers use Automatic Power Control
(APC) to maintain the output power constant over temperature and until the
batteries are nearly dead. The laser diode package itself has a built-in
photodiode behind the laser diode chip for this purpose. Only the very
cheap "dollar store" Far East import red pointers forgo this feature and
simply use a current limiting resistor - as the batteries discharge, the
output grows dimmer.

Most green laser pointers have in the past used Automatic Current Control
(ACC) - a constant current driver. The result is generally fluctuations
in output power as the pointer heats up. These may be quite large and
result in either a very dim spot or an excessive and illegal super bright
beam. The trend now is to use an APC driver to eliminate variability and
also make it harder to "boost" the output to an illegal and dangerous
power level.

Plastic versus glass optics: Plastic is softer than glass so if
the surface of the collimating lens is exposed, it will be more easily
damaged through carelessness and cleaning. However, since most of the time,
the lens is recessed, this isn't a major issue. What really matters is the
quality of the lens(es) and coatings, not what they are made of. Glass is
NOT inherently superior to platics. This affects the spot size and
definition, and the amount of ghosting and scatter. The only way to
know for sure is to check the beam for yourself.

Adjustable focusing lens: While this may at first appear to be
highly desirable, in the end it may turn out to be a nuisance going out of
focus on its own and prone to constant fiddling. (Of course, a piece of
adhesive tape or dab of glue can cure this malady.) However, make sure
that the pointer you acquire has had its focus properly set at the factory!
Even an apparently fixed focus model may have an internal ring that can be
turned with a paper clip once the front bezel is removed. Just don't
force anything and take care not to scratch the lens.

Multiple pattern generating optics: These are the sort of thing
that appear really neat and cute but in my opinion, have at most, limited
value. They reduce the overall brightness of the projected spot and except
for a basic arrow, just detract from a presentation.

Battery: The amount of time claimed for a set of batteries may tend
to be optimistic. Some/many/most may assume something about the usage pattern
in a pointing application (as opposed to cat teasing) like "25 percent on, 75
percent off". Red pointers are generally much much better in the battery life
department. The brightness of some pointers decreases significantly as the
batteries are drained which others remain exactly the same and then poop out
without warning. Expected cost of batteries will be affected by the battery
type - watch style button cells, AAA or N type alkalines, or expensive
lithium batteries. Although not as compact, pointers using common AAA cells
are probably the most economical in terms of battery replacement costs (and
AAAs are certainly the most readily available).

In general, it is best to remove the batteries if the pointer won't be used
for even a short time. Batteries have been known to leak and/or swell,
usually once they go dead. This is probably most likely to happen with
the cheap carbon-zinc cells provided as original equipment. It's virtually
impossible to salvage a pointer once such damage occurs because the cells
essentially wedge themselves in place as they expand. :(

Life expectancy and warranty: Sometimes there will be a spec like
"2,000 hour lifetime". This is probably mostly relevant for the expensive
green DPSS laser pointers and may be reasonable. Certainly, anything over
1,000 hours is adequate for a pointer used as a pointer within one's (human)
lifetime (or until it becomes obsolete). However, any lifetime claim isn't
of much value unless there is an enforceable warranty!

Fancy case: The polished hardwood case that comes with most green
pointers is really worse than useless. Dropping the pointer in the case will
likely render it useless. A cheap padded foam case would provide much
more protection. Red pointers are more robust but could still benefit from
pampering. :)

By now, you're probably totally confused. My advice: Use the specs for
guidance but if you really care about the quality of your laser pointer,
try a few out which come with money back no-questions-asked warranties and
keep the one you like. If, on the other hand, you just want to use the
pointer for presentations (what a concept!) and not to stroke your ego, the
cheapest red one will probably be just fine. :)

Some companies that sell laser pointers, rate them in terms of 'equivalent
brightness' compared to a 670 nm device. The Mark-I eyeball is about 7
times more sensitive to light at 635 nm compared to 670 nm. (Green laser
pointers at 532 nm will multiply this by another factor of 4 or 5.) (See the section:
Relative Visibility of Light at Various
Wavelengths.) For example, several of these companies offer laser pointers
with a '30 mW equivalent' output. This just means they are comparing a 635 nm
device optimistically to one of 670 nm. The actual output power is still less
than 5 mW. I do not really consider this deceptive marketing as long as the
meaning is understood. Here is a handy quick comparison chart for common and
not so common laser pointer wavelengths:

The term "Relative" refers to the visibility compared to the 555 nm peak of
human vision; the "factor" compares the brightness to that of an older 670 nm
pointer. Note that visual perception of brightness is not linear. Thus,
a 1 mW 532 nm green laser pointer isn't actually going to appear 28 times
brighter than a 1 mW 670 nm red model. What it means is that a 1 mW green
pointer will appear similar in brightness to a 28 mW 670 nm red one
(if such a thing existed).

As far as I know, CDRH approval will not be granted for any device of this
type over 5 mW actual beam power since their classification would then need to
be IIIb. So, don't expect to find a laser diode with an actual output power
of 30 mW in anything like a laser pointer! Frankly, I don't understand how
laser pointers with an output above 1 mW gain approval in any case. The 670
nm pointers especially (since they APPEAR less bright) represent a definite
hazard to vision at close range. Do not underestimate the stupidity of some
people who totally ignore all the safety warnings - "Wow, look at these cool
afterimages." - and then wonder why their vision never quite returns to
normal (though I do not know of any confirmed cases of irreversible damage
to vision even from this sort of abuse).

Another popular 'specification' is how far away the laser pointer is visible.
What the seller is probably actually referring to is the distance that their
Marketing department *thinks* the beam should be visible so long as this value
is greater than that of their competition. :-)

Seriously, who knows? There is no standards organization overseeing these
ratings. It could be the maximum distance to the screen that the beam is
visible:

to the person holding the pointer.

to someone near the screen looking at the screen.

to someone near the screen looking in the direction of the pointer.

Another consideration, of course, is whether this requires a moonless night!

Laser pointer marketers don't appear to have discovered (3) as yet (most
likely due to liability issues) since the number would be extremely
impressive - being in the many miles range! Apparently the Space Shuttle
astronauts were able to see a 5 mW red HeNe laser (632.8 nm, similar to the
best red laser pointers) from orbit, about 250 miles or 1.3 million feet.
Claims could be even more impressive for a green DPSS laser pointer (532 nm),
being about 5 times brighter for the same output power. Any marketing types
reading this? :)

Laser diode. This will have a 3 to 5 mW maximum output. The laser diodes
in older/cheaper laser pointers produced light at 670 nm (deep red).
Newer/better ones are in the 650 to 635 nm range (red to orange-red, which
appears many times brighter, mW for mW than 670 nm). Someday, we will see
inexpensive diode laser based laser pointers at all colors of the spectrum -
someday. Early laser pointers had the laser diode in its own 5 or 9 mm can
package mounted (probably press-fit) in a metal casting which in addition to
holding the optics (see below) acts as a heat sink. In an effort to reduce
costs, the newest and cheapest ones have the bare laser diode chip mounted
directly to a heatsink. You can forget about attempting to replace one of
these at home! In addition, they are likely not hermetically sealed so
contamination can easily enter from the battery compartment and degrade
diode performance or kill it entirely.

Power source. These are typical AAA Alkaline cells or watch-style button
cells. Depending on design, the battery must produce 1.5 V to 4.5 V or more.
In general, it isn't a good idea to substitute one type of battery for another
unless you know what effect it might have. With some, a dead pointer is the
most likely result. The battery holder may be a part of the case or a
separate unit. The on/off switch may be a simple spring contact or a high
quality enclosed type.

Power regulator. Many of the visible laser diodes used in laser pointers
have very precise current requirements. Too little and they don't lase; too
much and they turn into poor imitations of LEDs or die entirely. The only
reliable way of regulating the current for these is by monitoring the light
output. So, nearly all (if not totally all) laser pointers used to include
a laser diode driver that provides some degree of regulation based on optical
feedback from the monitor photodiode inside the laser diode package. The
better designs would maintain output power nearly constant until the batteries
were drained; the output power for some simpler designs would vary with
battery condition.

However, The Far East imports now flooding the market use only a resistor to
limit current - driving the laser diode just like an LED. The circuitry
consists of only 4 parts: laser diode, resistor, switch, battery. Apparently,
the type of laser diode they use has a wider operating range and can be driven
safely this way, though the output brightness will decrease as the batteries
are drained. See Components of Simplest Red Laser
Pointer and Closeup of Laser and Mount from
Simplest Laser Pointer. The inset in the first photo shows the laser
diode chip itself attached to a tiny metal block which is soldered directly to
the cast metal which acts as a heatsink. The top contact is a 1 mil gold
bonding wire.

Without the schematic there is no way to know how much protection is provided
by the driver. With some, the diode which can easily be
destroyed in an instant by using the wrong type of batteries, an external
power source (even one that you would think should work), or even putting
the batteries in backwards. The best designs will use a circuit that
regulates optical output based on feedback from the laser diode's built-in
monitor photodiode with respect to a fixed reference (voltage) and
maintain output power nearly constant under the battery is almost totally
drained.

On most pointers and diode laser modules, the laser diode driver is on a tiny
printed circuit board soldered directly to the leads of the laser diode
package. However, on some, the driver may be right next to the diode, sealed
in metal and look like part of the diode can, but isn't (possibly glued or
press-fit). This is likely the case if what appears to be the laser diode
only has two leads - all the visible laser diodes I know of come in 3 (or
possibly 4) lead packages to accommodate the monitor photodiode connections.

Collimating/correcting optics. At the very least, there must be a lens
to convert the highly divergent beam from the bare laser diode to one that is
roughly parallel. Better models will include optics to correct (at least
partially) for the laser diode's inherent wedge shape and astigmatism. Some
will also have an easy focus adjustment. In this case, a threaded barrel
(usually made of brass) holding the lens can be screwed in and out. A
spring presses on the lens to prevent unintentional movement and minimize
backlash. This is then sealed with something like Locktite or Glyptal. On
cheaper pointers, the lens or barrel will just be Epoxied in place with no
easy possibility of adjustment. Even if not an advertised feature, there may
be an internal ring that can be turned with a paper clip once the front bezel
is removed. Just don't force anything and take care not to scratch the lens.

Note that with the typical optics used in laser pointers, as much as 40%
or more of the light from the diode may be wasted largely due to its
high divergence in the fast axis (30 or 40 degrees total at the half power
point, perhaps twice this angle at the 10% point) - a very significant
fraction gets blocked by the small aperture of the collimating lens. I found
that an NVG D660-5 laser diode with an NVG collimating lens resulted in just
about a 50% loss between what was measured with the sensor of the laser power
meter against the diode's face capturing every photon compared to what ended
up in the collimated beam. I've been running one of these 5 mW diodes
continuously at a total output of 10 mW without any noticeable degradation.
With the addition of a microlens next to the laser diode chip, it would be
possible to capture a much higher percentage of the total light. With the
5 mW limit for laser pointers, this doesn't much matter but for other diode
laser applications, this would be beneficial. See the section:
Laser Diodes with Built-In Beam
Correction.

Some means of generating multiple patterns (optional). These permit the
projected shape to be selected to be something other than a formless spot
either by a built-in thumb-wheel type thingie or by replacing end-caps.

Photos of the internal components of typical red laser pointers can be found
in the Laser Equipment Gallery under
"Assorted Diode Lasers". The actual laser diode is not visible in any of
these being inside the brass cylinder next to the driver circuit board.

You've seen the Ads: "Laser Pointer with 42 Heads, $9.95.". These patterns may
be in the form of arrows or stars or a company's logo. They are either
built-in and selected by a thumb-wheel type arrangement or are in the form of
interchangeable tips that slip over the end of the pointer (as in the 'Ad'
above). There are 2 basic ways of accomplishing this:

Templates (stamped or photographically generated) in the form of the
pattern - basically a micro-sized slide. Since the beam from a laser pointer
is relatively well collimated or slightly diverging effectively originating
from a point source, anything placed in its path will be projected without
serious defocusing or degradation - over a modest distance at least. Thus,
the condensing and projection lenses of a slide projector are not needed.
This is the obvious and low tech approach but has at least two disadvantages:
Much of the available light is blocked by the solid parts of the template and
the edges of the pattern result in diffraction and interference effects which
DO degrade the projected image eventually. However, almost any graphic can
be produced equally well (or poorly depending on your perspective).

This type can be easily recognized because there will be a teeny-tiny replica
of its pattern visible by looking closely at the beam aperture.

Diffractive optics (also called a Holographic Optical Element or HOE).
A holographic process is used to produce a plate or film which when placed in
the laser pointer's beam, generates a pattern through interference just as
with a true hologram. These are more expensive to produce than simple stamped
patterns (at least in small quantities) and there may be some restrictions as
to the types of patterns (non-symmetric) that can be easily produced.

HOEs can be recognized by looking at them in normal lighting. What you will
see is: Absolutely Nothing. Or, at most, a dirty smudge, but no resemblance
to what results when used with the laser pointer.

Constructing your own pattern generating heads is probably not a realistic
option except perhaps for simple patterns using the template approach and even
that would be quite a challenge given the small diameter of the beam as it
leaves the pointer. Considering how cheap these things are now, it is also
probably not worth the effort unless it's something very special.

In my opinion, except possibly for an arrow, these things are really of little
practical value.

While the lasing line from a diode laser or even a cheap laser pointer is
quite narrow, there can be other wavelengths of incoherent light present in
the beam. Since the effective aperture of the laser diode is nearly a point
source (1x3 um typical), these spurious outputs will still collimate and/or
focus nearly as well as the laser beam itself. However, it's highly unlikely
that any of these are actual lasing lines except very near the main (design)
wavelength. No, you can't convert a red laser pointer into a rainbow pointer
with a simple modification performed on your kitchen table! :)

I've seen the existence of faint non-lasing light from more than one cheap
laser pointer as well as from a "dead" red laser pointer where the laser
diode had turned into an expensive LED. The orange, yellow, and green
output was of similar intensity to the same spurious colors present in the
lasing laser pointers so it is likely not related to high field intensities
when lasing but due to impurities resulting in non-red LED light.

To test for this (assuming you don't have an optical spectrum analyzer
handy), if the pointer doesn't have an adjustable focusing lens, use a weak
positive lens to focus the beam at a distance from the pointer of 0.5 to 1
meter - where the spot is still quite small, say less than 1 mm. Then, use
a diffraction grating (almost any will do including a CD or DVD) to view one
of these focused first order spots on a white card. Set things up so the spot
is either blocked or misses the card entirely so all you see is the area
towards the 0th order spot (undeflected beam). For my sample, there was a
continuous tail amounting to a few dozen nm. I couldn't quite tell if it
hit green but definitely was well into the yellow.

Another approach is to pass the beam of the pointer through a series of
mirrors that only transmit non-red wavelengths or reflect it from a series
of mirrors that only reflect non-red wavelengths. Using a pair of HeNe laser
resonator mirrors (an HR and OC in series) reduced the intensity of the red
wavelengths by a factor of about 100,000 so only a hand full of red photons
got through. :) This allowed me to clearly see the orange, yellow, and green
output of the laser pointer mentioned above by looking into the beam through a
diffraction grating. (Yes, this is safe once the red is filtered by the
two mirrors. It's just a dim glow and barely visible when projected
on a white screen in pitch blackness.) WARNING: Don't try the equivalent
experiment (looking into the filtered beam) with a DPSS (green or blue) laser
as there could be a significant amount of mostly invisible pump light at
around 808 nm that gets through to fry your eyeballs.

If you can power the pointer from an adjustable DC power supply (or have some
weak batteries), there may be an even easier way to see the non-lasing
colors - power the diode just below the lasing threshold. Under these
conditions, output at the lasing wavelength won't drown out the broad-band
LED emission and it will be easy to see its spectrum using any diffraction
grating or prism (or even through the edge of lens in a strong pair of
glasses!).

The use of the human eye apparently works a lot better than a fancy Optical
Spectrum Analyzer (OSA) because the intensity of the level for the non-lasing
wavelengths is so low and spread over a substantial range. The only thing
visible using an Ando OSA set to maximum sensitivity and averaging 10 times
was a slow increase in amplitude starting at about 566 nm and continuing to
the lasing wavelength of about 635 nm, but this wasn't even conclusively
above the noise floor for the instrument.

(From: Steve J. Quest (squest@att.net).)

The keyword here is you have a CHEAP laser pointer. I'm going to presume the
injection crystal lattice has contaminants in it, more likely if the
manufacturer also builds LEDs in the same factory. What you are getting
from your laser is a RED laser beam, and possibly green, orange, and yellow
LED light (non-coherent) which is also coming from the same crystal. Fire it
through a prism to see the various lines, I bet it's so polluted with foreign
dopants, that it produces a bright red coherent line, and a few non-coherent
red lines, an orange line, a yellow line, and a green line. That's all
possible since the injection diode crystal is basically an LED crystal with
perfectly cleaved ends, and a channeled electron injection pathway, axial to
the beam.

You can typically see this effect if you test the cheapest LEDs you can find
with a prism. I've found that dirt cheap green LEDs usually produce both a
green and a yellow line. Dirt cheap reds produce several lines of red.
You can get many wavelengths out of a gallium arsenide crystal.

Red laser pointers are by far the most common and now quite inexpensive.
Pretty soon, they will be given away free in specially marked boxes of corn
flakes. :) Seriously, prices under $5 aren't uncommon and dropping rapidly.
Search on eBay and you'll probably find them for less than $1 each in bulk.
However, except for various shades of red (depending on wavelength), all
other colors are very expensive. In fact, there is really only one other
color of any practical consequence - green. And this is a much different
type of laser than the simple diode lasers used in red laser pointers.

Currently, nearly all green laser pointers are based on Diode Pumped Solid
State Frequency Doubled (DPSSFD) laser technology. They are not just red
laser pointers with a different laser diode or green lens! (See the section:
Diode Pumped Solid State Lasers.)

The exceptions are older models using green helium-neon (HeNe) lasers. I bet
you didn't know HeNe lasers came in green, huh? :) These had power outputs of
much less than 1 mW and were very bulky compared to modern laser pointers. And
while green HeNe lasers and even relatively small green HeNe lasers that
could be used for laser pointers - are still manufactured, actually
using them for pointing is about as common as finding raw dinosaur eggs.
(See the section: HeNe Tubes of a Different
Color if you are curious.)

The wavelength of the DPSSFD lasers is 532 nm based on the intracavity
frequency doubling of a Nd:YVO4 (vanadate) chip using a Potassium
Titanyl Phosphate, KTiOPO4 (KTP) crystal inside the laser cavity.
Their output may either be CW, quasi-CW, or pulsed. CW means "continuous
wave" which results in a constant intensity spot. Quasi-CW and pulsed both
result in a spot that varies in intensity (so they are really both pulsed
output) but the pulses for the quasi-CW variety may be at a much higher
frequency (e.g., 5 kHz versus 300 Hz). You can tell which you have by moving
the spot rapidly across a screen - the trace from the quasi-CW and pulsed
types will break into discrete spots. However, the spot spacing for the
quasi-CW pointers may be so small for normal use that for all intents and
purposes, they will appear continuous. However, a quasi-CW pointer would
not be a good choice to use in a laser show application. (Note that there
is no standard for calling a particular pointer quasi-CW or pulsed so your
advertising blurb mileage may vary!)

Visibility of these green pointers is 4 to 5 times that of 635 nm diode lasers
or 632.8 nm red HeNe lasers, which in turn appear 6 or 7 times brighter than
the older 670 nm laser diode based laser pointers for the same power output.
The maximum legal green laser pointer power is still only 5 mW but this would
be equivalent in brightness to something like a 150 mW, 670 nm device! And,
the sellers of these things don't let you forget it! :)

Battery life of any green pointer is likely to be much worse than that of
the simpler red variety though for actual uses as a *pointer* (what a
concept!), it probably doesn't matter all that much. The quasi-CW and pulsed
variety should be somewhat better in this regard. (The "spec" sheet that
comes with the Edmund Scientific L54-101 green laser pointer claims a 3 to 4
hour battery life from a CR2 lithium cell though I'm not sure I believe it.)
There is no functional advantage to the pulsed system (it's actually less
desirable since the spot breaks up into dots when swept over a screen) but it
can be made much more efficient reducing the need for thermal management and
extending battery life at the same perceived brightness for these current
hogs. Quasi-CW (frequency in the kHz range) pointers may use either a pulsed
pump diode, a passive Q-switch (sometimes called FRQS - Free Running
Q-Switch), or both, to improve the efficiency. Pulsed pointers (frequency
in the hundreds of Hz range or less) use a pulsed diode.

Note that since there is no real control of temperature, power output may
change significantly (up or down or both) for pointers using a constant
current driver, also called Automatic Current Control (ACC) if the
pointer is kept on for an extended period of time. Most pointers have
used ACC drivers. Usually, since pointers are really intended to be
used for brief periods of time for pointing at something, if any optimization
was done, the manufacturer would attempt to select the laser diode wavelength
to match the vanadate's absorption band when the components are cool.
As the laser diode heats up, its wavelength increases (about 0.3 nm/°C)
and drifts away from the optimal value. (Even though the absorption band is
quite broad, there may still be some noticeable effect.) However, if the
wavelength was low to begin with, the power would increase as the wavelength
moved toward the peak absorption for the crystal and then decrease if it went
far enough. From my experience with these as well as other basic green DPSS
lasers, unlike red laser pointers whose output is either constant or gradually
dropping in intensity until the batteries poop out, expect a modest amount of
slow cyclical and even possibly some sudden power fluctuations as the
temperature of key components increase and lasing characteristics change.
So, a typical green pointer may actually dip to less than 2/3rds of its
rated power at times, hitting the rated power only occasionally. Apparently,
many may significantly exceed the rated power (and the legal limit) at
times if you happen to get lucky or unlucky, depending on your wishes.
Some of the newest green pointers use Automatic Power Control (APC) both to
get around the variability and excessive illegal power problems. An angled
plate feeds a small portion of the output beam to a photodiode are used
in a feedback circuit to maintain the output power constant until
the batteries die. Some may even seal the entire driver in hard Epoxy
or at least the power adjustment pot (if there is one) to make it more
difficult to "boost" the output power above the legal limit as some
people want.

And don't forget that just because the CDRH safety
sticker may say 5 mW max, your actual model may not come anywhere near that -
ever. The actual power rating would be listed elsewhere. But
providing it at all is rare, partially due to the fluctuation problem, but
mostly because the manufacturers figure you're better off not knowing how
mediocre the pointer realy is!

With the much higher prices for green pointers (at least in the past!),
make sure you get a decent
written warranty. No, I really can't recommend a particular manufacturer or
model. I'd suggest checking the archives of the usenet newsgroup
alt.lasers via Google Groups for
recent discussions the best green laser pointers to buy.
Prices are currently averaging about $250 (in 2001) though
I've seen some 3 mW models advertized on the Web for as little as $180,
lower on eBay). And supposedly, though I haven't tried to buy one, there
is at least one company (Leadlight
Technology, Inc., Taiwan) who will sell 1 to 3 mW green pointers for
as low as $88, quantity 1 (probably even lower by now). And, I've seen
Chinese imports going for under $20, including shipping! (Summer, 2007)

Although some may
consider it unethical, ordering several pointers and only keeping the best
may be the only way to assure satisfactory performance as they are quite
variable in output and stability. The additional complexity and more
delicate nature of the individual components means that reliability and
robustness may not be as good as for their red cousins (to the extent that
these are reliable and robust!). This means that while those fancy polished
wood cases look impressive, transporting the pointer in a well padded case is
probably a better idea. Comparing the detailed diagrams of a
Typical Red Laser Pointer and
the Edmund Scientific L54-101 Green DPSS Laser
Pointer, or the single diagram Comparison of Red
and Green Laser Pointer Complexity. (The L54-101 was a $395 model
around 2002, but even so, it's amazing prices weren't a lot higher as
it has all the sophistication of a much more expensive DPSS laser.)
Even a failed switch just out of warranty (assuming there is a warranty
that will be honored in the first place!), can render a $300 pointer
useless since there is often no non-destructive way of getting inside
to repair it. (And, I've heard that the switches they use on these
things are often not adequately rated for the much higher current
green laser pointers use compared to red ones.) Of course, now (2008),
presentation-power class green lsaer pointers (i.e., 5 mW)
are more along the lines of $10 or $20, so a warranty might be luxury
from a bygone era. :) They also use composite crystals instead of
discrete crystals so the complexity is somewhat lower as shown in
Typical Green DPSS Laser Pointer Using MCA.

For more information on DPSS lasers and green laser pointers including details
of the L54-101, see the sections starting with:
Diode Pumped Solid State Lasers.

And, what about those other colors? As a practical matter, there isn't much
need for anything beyond green since its wavelength (532 nm) is near the peak
(555 nm) of the human eye's response curve. However, to impress those high
flying corporate executives, blue might be cool - but expect to spend a
$2,000 for one using DPSSFD technology that isn't as bright as a $5 red
pointer. I think yellow would look nice on dark color slides, but the only
way to do this until recently would be to use a yellow HeNe laser (yep, they
come in yellow also!) as there are no yellow laser diodes. However, at least
one company is now offering what they claim to be a yellow DPSS laser pointer.
See Laser Glow. No real data
available though. It apparently uses sum-frequency mixing of the two
strongest lasing lines of Nd:YVO4. The sum of the frequencies
for 1064 nm and 1342 nm corresponds to the listed 593.5 nm wavelength.
(1/1342+1/1064=1/593.5.) So, they have the laser running simultaneously
at the two wavelengths by suitably coating the mirrors and use a non-linear
crystal (probably could be KTP) phase matched to do the summing.
Cute how the physics happens to work out. :) Anyone volunteer to buy one?
See U.S. Patent #5,802,086: Single Cavity Solid State Laser with Intracavity
Optical Frequency Mixing.

Orange is a similar problem but there is no vanadate lasing line at a suitable
wavelength with adequate gain. At the other end of the spectrum, violet
(which would be really hard to see) laser pointers using the Nichia violet
(400 to 415 nm) laser diodes could be built inexpensively like red ones since
the circuitry is about as simple - except for one minor detail: the cost of
these violet laser diodes is presently (February, 2001) still around $1,000
each! A violet pointer might impress the corporate big-wigs also but due to
the lack of visibility, would be quite useless for presentations unless the
projection screen had a coating that glowed when hit by violet light. Hmmm,
now that's an idea. :)

There are inexpensive LED-based key chain pointers in bright blue and other
colors but these are not true lasers and the divergence is typically 5 to 10
degrees instead of 1 or 2 milliradians (1 degree = 17 mR). But, if
all you want to do is impress management types, that may be good enough. :)

And, no, there is currently no technology capable of producing a variable
color laser pointer.

So, now you should know the reasons that the only way to convert a red laser
pointer into a green one is to buy a bunch of red pointers for a low price,
sell them for a high price, and use the proceeds to purchase a green laser
pointer. :)

Unfortunately, these usually don't come with any sort of useful user manual.

Much of the following applies to any laser pointer but especially to the
expensive green variety:

Avoid physical shock: Due to the additional complexity and precise
component alignment, green DPSS laser pointers are much more susceptible
to damage from being dropped or wacked on a hard surface.

Use ONLY the recommended power source: Substituting a different
battery type or external power supply may result in immediate damage or
reduced life. Without analyzing the driver circuit, there is no way to
know for sure what is safe.

Do NOT attempt to operate if very cold: The pump output will be
greater and the components and adhesives will be more brittle. I don't know
how to determine whether your pointer will be affected and at what precise
temperature. However, I've heard of the vanadate being shattered from
operation at low temperatures.

Do NOT attempt to disassemble the pointer: At least, not if you
value it as a working pointer. While getting at the driver and DPSS module
may be relatively low risk, taking the DPSS module apart is definitely to
be avoided. Damage is almost certain.

Do NOT remove the IR filter: A substantial amount of the 808 nm
pump light can leak through to the output (10s of mW or more which is
potentially dangerous) and this is nearly totally invisible and swamped by
the green output. Some totally invisible IR at 1,064 nm from the vanadate
lasing process also leaks through the output mirror and its power may be
similar to that of the green output! The IR filter is typically a greenish
piece of glass buried inside the collimating optics but on some models,
unscrewing the end-cap also removes the IR filter. The risk of the
additional IR isn't worth the very slightly increased green output that
may result. It's best to avoid these models entirely or somehow make sure
that the end-cap can never be left off accidentally.

Some/many imported green pointers don't even have IR filters. This
of course is a serious safety hazard. But, it also may result in bogus
readings for green output power. So, someone can claim "300 mW from this
green pointer" without saying that 299 of those mW are IR! :)

If you have a green laser pointer and some means of detecting IR, the 1064 nm
beam will be almost in the same position and with similar collimation to the
532 nm beam. However, the difference in wavelength will result in a change in
effective focus/divergence and there is a 4.5 mR walkoff from the KTP of
532 nm beam with respect to the 1064 nm beam which will result in some
discrepency in where they point. The 808 nm beam will be highly
divergent/diffuse but may be quite intense next to the output. Note
that an IR detector card will likely fluoresce due to the energetic
532 nm light so a glow in the area of the beam itself is not
necessarily an indication of serious IR leakage.

Do NOT attempt any modifications to the driver: While it is tempting
to tweak up the drive current just a bit, or replace the pulsed driver with
a CW one to get more power, my recommendation is to avoid any of this unless
you won't mind turning your sleek green pointer into a sleek piece of junk.

To improve reliability and extend operating time, it may be possible to mount
the guts of a green pointer in a different case.
Here's an example of the module from a green DPSS laser pointer that has
been repackaged by Dave (ws407c@aol.com) with enhancements by me (Sam)
into a little blue box. Improvements include the use of AA instead of
AAA batteries, a better power switch, a cushioned mounting for the DPSS
module, and some genuine safety stickers.
See: Green DPSS Laser Pointer Module Mounted in Little
Blue Box. For those contemplating doing what I recommend against, this
makes it easier to access the adjustment pot as well. :)

You've probably seen the advertisements or eBay listings by now - or perhaps
you already own one - something along the lines of "OEM 60 mW Green DPSS Laser
Pointer". Technically, this may be possible with some units, at least if you
don't care about stability, battery consumption, and short (possibly very
short!) lifetime, but how legal is it if the output power is actually above
5 mW which is supposed to be the maximum for any pointer available to the
general public? The short answer is: It's not legal at all. In fact, were
you to purchase one of these, even if it came anywhere close to the claimed
power (how many buyers actually have a laser power meter to check?!),
the CDRH sticker will probably still say "<5 mW". So if questioned,
perhaps the seller will say either that it is only for incorporation into a
product (thus the "OEM" which stands for "Original Equipment Manufacturer")
or that the higher power must have been the result of shipping damage.
Right. :)

Being able to significantly increase output power with an adjustment or
simple circuit modification only applies to green pointers. Red ones will
just die if this is attempted much beyond 5 mW - a higher power laser diode
would be needed.

Note that as a matter of principle, I do not have detailed information on
boosting the output power of a laser pointer above 5 mW anywhere in this
document due to the fact that (1) it is illegal, (2) it is dangerous for the
user and others, and (3) any adjustment or modification is quite likely to
destroy the pointer or at least dramatically shorten its life. However,
there is plenty of such info available on the Internet. Use at your own risk.

As a practical matter, most of the pointers sold with an output power of
significantly more than 5 mW have either simply had their diode current
turned up, or had the diode replaced with a higher maximum power device.
In both cases, the lasing crystals are likely being overstressed and
inadequately cooled. A rapid degradation or total failure is quite possible.
These are not $10,000 lab lasers, but $50 pointers on steroids. Good luck
on getting warranty service. :)

In order to become more compliant with CDRH regulations, manufacturers are
being forced to modify their designs to assure that the output power never
exceeds the 5 mW limit at any time under any conditions, and to make it more
difficult for any modification to be performed that would violate the 5 mW
limit. These techniques include eliminating any internal adjustments,
potting the driver circuitry in Epoxy, converting from a constant current
to a constant power driver, and using components that are funning closer
to their rated specifications.

For anyone considering the purchase or sale of a modified laser pointer,
here are a list of guidelines. This applies to any color pointer as long
as it's based on a laser:

Laser pointers sold in (or to) the USA must have a power output
of less than 5 mW.

Using a modified laser pointer with a power output greater than 5 mW
in public without a variance is a violation of CDRH rules.

Selling a modified laser pointer with a power output greater than
5 mW is also a violation of CDRH rules. Some reputable companies now
list pointers with a power output of 50 mW or more, claiming that because
they have a keyswitch, emission indicator, and shutter, they are legal.
Unless the CDRH rules have changed recently - which I highly doubt given
the current climate with respect to terrorism and litigation - this doesn't
help.

Modifying a laser pointer so that it is capable of more than 5 mW
output via a user accessible adjustment, but only selling it adjusted
at less than 5 mW is still a violation of CDRH rules.

Just calling a modified pointer an OEM device does not clear the seller
of requirements to follow CDRH rules.

While it would seem that despite the proliferation of modified green laser
pointers, any violations have thus far fallen below the threshold for action
by the CDRH, it won't take too many law suits to change this!

So, aside from bragging rights on having the most powerful laser pointer
on your block, what use are they?

Astronomy types claim they are better for pointing out stars and other
celestial bodies in a dark sky since the actual beam path is more visible.
However, 5 mW really should be adequate, if not ideal.

There may be some forensic applications, though lasers designed with
the appropriate safeguards and regulatory approvals exist for this purpose.

They don't make good flashlights and are useless or worse as defensive
weapons. A $3 flashlight produces more light than the most boosted pointer
and trying to use a laser of this type type against an armed intruder will
probably result in a annoying hole appearing in some critical part of your
anatomy.

And due to their poor performance, boosted pointers are worthless for
laser show or any other similar application.

So that leaves bragging rights and personal satisfaction. There is
nothing illegal about admiring the photons in the privacy of your living
room. But a high power hand-held laser with a simple pushbutton switch
is a truly dangerous toy even for the user. And if someone else should
ever suspect they were injured by the pointer - whether valid or not -
the lawyers will pounce!

(From: "Lynn Strickland" (stricks760@earthlink.net).)

A hand-held pointer over 5 mW is illegal to sell in the USA, period. Regs
per IEC 825 in Europe are even tougher. The CDRH hasn't caught up with
everyone yet, but the fines are big, and they'll force a product recall. (If
you don't have records of who bought your product, ship dates and serial
numbers, you've got a second problem.) Even pointers under 5 mW require a
"variance" document with respect to certain CDRH regulations, and require a
CDRH accession number.

Calling it an OEM product (with disclaimer of non-compliance) still doesn't
fly, because the law applies to any "removable laser system." The only time
you can sell a non-compliant removable system is when you can site the
purchasers CDRH accession number for the end product.

Claiming "shipping damage that resulted in increased power" also doesn't
fly, because CDRH regs require designs in which system failure cannot
result in exceeding the specified classification.

Some have sold the laser 'head' and 'power supply' separately as a kit. If
one can reasonably attach the pieces without specialist tools, etc. -- even
the KIT has to comply (and be certified, and have an accession number).

Having lived with these laws as a manufacturer, I can tell you that there
aren't any cute and clever loopholes. Sooner or later they'll get your
number. People will show up at your door and start packing your files and
PCs into the back of a white van with government plates, and you'll be
calling your attorney from your car phone, because they won't let you back
into your office. It's like export regs, you can fly under the radar screen
for a while, but once they find you...

If you want to screw with the companies selling this stuff, ask for the CDRH
accession number for the product in question, along with any variance
numbers under which they are shipping the specified product.

(From: Steve Roberts (osteven@akrobiz.com).)

People who do not register as a manufacturer and who don't do a "product
report" and the import paperwork get clobbered big time. I know a fellow
who had $10K in legal bills for selling an "OEM part" without the stickers
and filing the reports.

It's not just a variance for most pointers, it's a manufacturer's initial
report, yearly report, and record keeping, very good record keeping,
for 7 years or so.

Now that Customs and CDRH are paired up, things are getting regularly
stopped, they publish a on line list of seizures from time to time
and its very long! And it isn't just little guys who get seized, there
are some serious big time companies who have problems.

What's illegal about the hopped up "OEM DEVICE" pointer is entering it
into commerce under (1) the illusion that the buyer will make/keep it
compliant and do any paperwork before reselling it and (2) that it's
entering into trade to someone who will not use it for its intended
purpose as a certified Class IIIa demonstration device. If they use it
in public when modified, then it's illegal. If it's sold with intent
to modify it to beat the rules, then thats also illegal.

Ironically, many newer cheap laser pointers can be modulated at very high
rates by simply controlling the current from the batteries/power supply. Why?
Because they don't have any power regulation and the super cheap Far
East imports have no filter capacitors at all. Of course, you risk
blowing the laser diode if this isn't done carefully. But, for the
typical pointer using 3, 1.5 V button cells, just feed it with a signal
clamped between 0 V (or around the 3 V lasing threshold) and +4.5 V capable
of supplying around 50 mA and it should be possible to generate a modulated
output up into the 100s of MHz range. Use a frequency modulated carrier for
best audio or video performance. See the additional comments below.

If all you want to do is adjust the power manually, just add a 100 ohm pot
in series with the battery. On my tests of typical cheapo pointers, that
varies the power from just below lasing threshold to maximum. Note that
the beam from LED emission below threshold is dim but still quite decent
in terms of divergence so it may be acceptable for applications that don't
require the narrow line width or coherence of a laser.

On those that do have decent regulators, modulation frequency may be limited
to a few Hz to a few hundred Hz depending on design and the actual output
power may be more of a triangular wave shape due to the soft start (ramp up,
ramp down) turn on, turn off behavior.

(From: John, K3PGP (k3pgp@qsl.net).)

The speed issue was true of many early (and pricey!) laser pointers which
used a feedback power regulator. The capacitors and the feedback tended to
reduce the speed at which the laser could be turned on and off.

Now that the price has fallen everyone is competing to make them even cheaper.
What this means is that most laser pointers today have NO power regulator at
all. What I've been finding is a laser diode, resistor, switch, and two 1.5
volt batteries in series. Laser pointers like these can be modulated up into
the hundreds of Mhz as there is nothing to interfere with the speed at which
the laser can be turned on and off.

Of course you stand the risk of easily damaging the diode in laser pointers
like these with an overvoltage, spike, or static electricity if you don't
use some common sense and are not careful when bringing wires out and hooking
the laser pen to external circuitry.

Since we are dealing with a wide variety of styles and manufacturers, there
will be some differences. For instance I've seen a few that have no power
regulator, just a resistor to the 3 volt battery supply, BUT have an
electrolytic capacitor across the diode. It was necessary to remove the
capacitor to allow the laser to be switched at high speed.

A collimated diode laser module and pocket laser pointer both produce a spot
of light. So why the typical huge difference in price?

The simple answer is: It all depends. :) There can be variability in any type
of product. While the desired output of a laser pointer and collimated diode
laser module is similar, how fussy the end-user is and how one gets there may
not be:

Laser pointers are mass produced so this helps reduce costs. They generally
have less complex and less robust drive electronics since the power source
is supposedly well defined - a set of batteries. There may be no corrective
optics for the astigmatism and elliptical aberrations of the typical laser
diode - at a distance, your spot isn't a nice round Gaussian profile. There
is probably just a single cheap plastic lens glued in place, though some
models do have adjustable focus.

Diode laser modules are more of a specialty item used inside other
equipment and for optics research and development. Production volumes are
not as high. They usually (but not always) have high quality driver circuits
designed to protect the sensitive laser diode from moderate abuse - noisy
power, for example. Many have high quality optics including additional
elements for correction of the laser diode aberrations. They usually have
adjustable focus.

In the end, it is probably the mass production that is the most significant
factor in keeping costs down.

There is also another difference between the two which relates to output power:

For a laser pointer, the power rating - if it can be believed at all - is
a maximum you might see with fresh batteries under the right conditions on a
very good day, or possibly just the 5 mW maximum for Class IIIa (which is the
most that is legal in the USA for a laser pointer). Obviously, the seller
wants to impress you with the specs for their product and not all are being
entirely honest or forthcoming. The actual power could be much less and may
decrease rapidly as the batteries are drained.

For a diode laser module (from a reputable manufacturer at least), the
power rating is likely to either be what they actually measured for that
sample or a guaranteed minimum value after warmup (power goes down with
increasing laser diode temperature), or after some number of hours of
operation. Thus, the actual output power could be much more under some set
of conditions. The CDRH safety sticker will still list an upper bound but it
will likely be much higher than the module's power rating.

Unless you find a really good deal on excess inventory or the like, the guts
of laser pointers are probably the cheapest source of decent quality diode
laser modules for many applications. These are mass produced so cost can be
quite low. There are many suppliers who will sell you just the laser diode in
a brass mount with adjustable collimating lens and attached driver circuit on
a tiny PCB for under $10 for a single unit, less in larger quantities.

These aren't likely to be in the same league as the $300 diode laser modules
from Edmund Scientific or even $100 units from other sources which will meet
or exceed all specifications and have protection against all reasonable abuse,
for the price, they can't be beat!

With respect to specifications:

At rated voltage, they may not put out more than 2 or 3 mW, maybe less,
even if 5 mW is claimed. But, you may be able to push them higher by
tweaking the on-board pot some provide, or by running on slightly higher
input voltage. CAUTION: your mileage may vary and you risk blowing out the
laser diode or driver - See the additional information in the section starting
with: Diode Laser Modules and Laser
Pointers. (These may not have a driver that regulates for input voltage
though most do use the monitor photodiode for optical feedback power control.)

The wavelength may be optimistic - somewhat longer (more towards the red/IR
and less bright than advertised). For example, I measured one model that
claimed 650 nm but was actually around 657 nm.

So you found a bag of cute little brass devices marked 'barcode lasers' at a
garage sale. They have wires coming out of one end and a lens at the other.
Are they bare laser diodes or do they have a built in driver circuit? Size
alone is no real indication as the driver circuits can be quite tiny.

If there is any sort of model number, try to check that out first since as
we have seen, laser diodes are not very forgiving of even minor abuse.

If you really have a *bag* of the thingies, the surest way to know what is
inside is to sacrifice one and reverse engineer it - unless, of course, they
are totally potted in Epoxy or something even more impervious to 20th century
technology. Perhaps your friendly dentist would be willing to X-ray one
for you (with or without root canal). :)

Assuming that analyzing the circuit isn't possible or appealing and they are
not clearly labeled (in which case you wouldn't be reading this anyhow),
closely examine the wire leads:

If there are three solid gold plated wires and they connect directly to the
bottom of what looks like a metal can transistor, you have a bare laser diode.
This sort of diode laser module without driver circuitry is common in
equipment like laser printers and barcode scanners. Some laser diodes only
have two wires but not the kind you are likely to run across in a grab bag
unless one broke off. :-) If you cannot tell where they go, also assume you
have a bare laser diode. See the chapter: Laser
Diode Power Supplies to determine their electrical characteristics and
power requirements

If there are only two wires, probably stranded and color coded red and
black, there is almost certainly an internal driver circuit. Red will be
positive. A typical power requirement is 3 to 12 VDC at 100 mA. Start
low while monitoring output (using an IR detector if it may be an IR laser
diode). Most diode laser modules operate at a fixed maximum output power
which will be where the intensity stops increasing as you raise input voltage.
You can probably assume the circuit will handle, say, 25 percent more input
voltage beyond this point but there is no way to how much more without
reverse engineering or testing the unit to destruction. The power curve will
also probably be quite non-linear - starting out as an LED until the
threshold current is reached and then increasing rapidly.

CAUTION: Some diode laser modules are current controlled using optical
feedback but expect a regulated DC power supply input. With these, the
output will continue to increase more or less linearly as the input is
cranked up until the point at which the smoke comes out. :-(

If there are three (or more) stranded color coded wires, the additional
ones may be for on-off control, modulation, or a shield or ground. Have fun
determining exactly what they do (but be prepared for frustration).

The maximum legal limit for power output from any laser pointer in the USA
is 5 mW - Class IIIa (there may also be more restrictive local regulations and
it's lower in some other countries). The best color to use is green since the
wavelength of modern green laser pointers based on Diode Pumped Solid State
(DPSS) laser technology (532 nm) is very near the peak of human visual
sensitivity (555 nm). Thus, a 5 mW green laser pointer produces nearly the
brightest beam allowable by law (about 0.9 relative to 555 nm). (Although
older green laser pointers based on green helium-neon lasers were a bit
closer at 543.5 nm, one capable of 5 mW would be almost a meter long and
weigh several kilograms with the required backpack mounted battery and high
voltage power supply.) Whether the beam is pulsed or continuous doesn't make
much difference. However, the spot from a low divergence beam may be somewhat
more visible at a distance on a brightly illuminated surface (see below).
The difference between a 4 or 5 mW pointer isn't really that significant
(it's barely detectable even with two pointers side-by-side), and as a
practical matter due to the technology, output may vary by as much as 30
percent (up, down, or in a cycle) as components heat during use.

So, if even 5 mW of green isn't bright enough, the optimal solution would be
to control the ambient illumination by putting a dimmer on the Sun. :) If
this isn't an affordable option, the best that can be done is to use a screen
or whatever that is a light color and has a diffuse surface, and orient it to
avoid direct Sunlight. Unfortunately, if there is no way to control any of
this as would be the case with use by an outdoor tour guide, there are no good
solutions. Even the best laser pointers have a divergence no better than
about 1 milliradian (1 part in 1,000) so the power density of a 5 mW green
spot projected on a surface more than a few meters away drops well below
that of the 0.5 to 1 mW per square millimeter of Sunlight. Even the pure
green color of the laser pointer will be quickly overwhelmed by the ambient
illumination.

I know that in your fantasies, you have dreamed about the possibility of
creating a burning laser or Star Wars style light saber from a laser pointer.
Unfortunately, neither of these is even possible theoretically. The best you
could ever hope for would be to obtain at most 5 mW from a device currently
outputting 2 or 3 mW.

While it might be feasible to increase the current to the laser diode, unless
you know its specifications AND have an accurate laser power meter (mucho $$$),
there is no way of knowing when to quit. Above their rated maximum optical
power, laser diodes turn into DELDs (Dark Emitting Laser Diodes) or expensive
LEDs. Exceed this rating for even a microsecond and your whimpy 3 mW output
may be boosted to precisely 0.0 mW. This is called Catastrophic Optical
Damage (COD) to the microscopic end-facets of the laser diode. There can be
also be thermal runaway problems or a combination of both of these depending
on design - or lack thereof. However, if you have a bag of these gadgets and
are willing to blow a few, here are some guidelines:

Diode laser modules almost always have an internal current regulator using
optical feedback to stabilize power output. With these, the only hope of
changing the output power is to get at, and adjust or modify this regulator
circuitry. If the unit is fully potted, you can basically forget about doing
this. If the driver board is accessible, there may be a control to adjust or
resistor to change that sets power output. See the chapter:
Diode Laser Power Supplies for more details
and sample circuits.

Laser pointers may or may not have an internal regulator. There is no way
of telling from physical appearance and the pedigree of a typical laser
pointer is often unknown - even if sold by a reputable dealer. Price may be
an indication (more expensive may imply better circuitry) but there no way to
know for sure. However, a simple test or observation can probably determine
this:

If the brightness of the laser beam (the spot on a wall - don't look into
it!) is IDENTICAL whether you use a fresh set of batteries or ones that are
slightly used, it also has an internal regulator and the comments above with
respect to diode laser modules apply. (I am not referring to normal decrease
in brightness when the batteries become nearly dead - but obvious changes
even near their beginning of life.)

Where there is an internal regulator and adjustment pot, turning it may
increases the brightness initially. However, as the laser diode heats up
over a few seconds or minutes, its output with respect to current decreases
and the regulator will keep increasing the current to compensate - a runaway
condition which can also result in damage or death to the laser diode. A
large heat sink, active (e.g., Peltier or heat-pipe) cooling, or dunking in
liquid nitrogen may help if you are really determined to get every last
photon from your laser pointer or diode laser module! :) (I've heard of
people getting truly spectacular amounts of light out of laser diodes cooled
to liquid nitrogen temperatures, at least for a short time.)

If, on the other hand, brightness is a strong function of the battery age
and type, even for nearly new cells, there is probably no internal regulator
and using a **slightly** higher voltage power source **may** increase optical
power without blowing the laser diode. Since there is no regulator, what
circuitry is present, was probably hand trimmed or adjusted for the
particular laser diode so that a fresh set of the recommended batteries
produce a safe (from the point of view of the laser diode) output level.
However, there may be some margin to play with - or there may not. If you
feel lucky, go for it!

However, another risk, is that after having painstakingly set the current
or resistance or whatever for a brighter output, the next time you turn it
on, the laser diode may blow! The reason is that when cold, as noted above,
the optical output of a laser diode is greater for a given current and may
exceed what the laser diode facets can tolerate even if it was well within
safe limits with the laser diode warmed up. With no optical feedback, there
is no protection against this possibility

But, in any case, how will you know when to quit before the laser diode is
irreversibly damaged? And, in addition to exceeding the maximum rated output
power as you crank up the laser diode current, an electrostatic discharge, a
voltage spike from an external power supply, a noisy power adjust pot, or the
phase of the moon on an alternate Tuesday, may be enough to blow it! By the
time you notice a problem, it will likely be far too late for the health of
your poor little defenseless laser diode!

This really IS like playing Russian Roulette and my serious recommendation
would be to leave well enough alone. Save for a more powerful unit or even
just a 635 nm laser pointer if your current model is 670 nm (which will appear
at least 5 times brighter for the same output power).

If you do insist on modifying the circuitry, use an antistatic wrist strap,
grounded temperature controlled soldering iron, and the proper desoldering
equipment (if needed). At least then, you'll know that it was more likely
the changes to the circuit that blew out the laser diode, not your rework
technique. :)

The same basic comments apply to boosting the output power of expensive green
laser pointers (but of course there is much more to lose). The adjustment
may vary current or for those that are pulsed (which are most of them),
the duty cycle instead. With no thermal management, stability is likely to
be significantly worse at higher power even if the laser diode survives.
However, since 3 mW and 5 mW pointers may be physically identical inside and
out, I don't know whether they are sorted on the basis of power output before
labeling or is just a matter of the setting of the power adjust pot - it
probably depends on manufacturer/model.

Having said that, I've heard of this being successful and I've also heard of
at least one sample of a green laser pointer producing 36 mW out of the box. :)
The vanadate/KTP crystals should be capable of much more than 5 mW, at least
for awhile. However, in the samples I've seen, the discrete vanadate is
mounted by just two tiny dabs of adhesive which could easily come unglued
if the crystal gets hot (which it would with higher pump power). Green
pointers using composite (e.g., CASIX) crystals would eventually suffer from
the dark spot problem in the glue used to hold them together. There are
instances of very "lively" pointers where just tweaking the OC mirror could
result in increased power if not optimally adjusted originally. I'd consider
this the exception though. Most likely, boosting power would require
higher current to the pump diode which will result in shorter life or no
life at all!

(From: HippyLaserTek (hippylasertek@aol.com).)

Since the switch died in my green pointer, I said what the hell, and gave it a
shot. (For crying out loud, why don't they replace the switch with a soft
touch type like in a calculator and a saturation driven transistor! Hell at
$200 to $300 a pop that's the LEAST they can do!)

Well I didn't expect 50 mW out at reasonable currents but I DID get around 15
mW of green out just by carefully tweaking on the three setscrews which
adjust the OC mirror position. The only sacrifice was a slight decrease in
beam quality so it looks oval instead of round, but for a pointer module, who
cares anyway.

It was cool not only seeing that kind of power from the pointer, but the mode
patterns as well were rather interesting too. Some of the patterns were very
beautiful. By turning the current up from it's original 400 mA to 450 mA, it
topped 25 mW, the max my low power laser meter reads! It's rated for HeNe
light, so i don't think it responds the same for green. I think it gives a
false low reading though, I KNOW it does for blue. (This is true for a
typical silicon photodiode, possibly as much as 20 to 25 percent reduction
at 532 nm compared to 632.8 nm. --- Sam.)

Going the other way I got green threshold at a mere 140 mA and "rated power"
of 4.8 mW at around 250 mA. I'd LOVE to install a 2 watter pump diode in place
of the 0.5 W? (tested at 0.4 W at 400 mA on my Ophir power meter set on
shg/dye/argon setting) pump diode in it. I am fairly certain with that diode
pumping the DPSS laser guts it would EASILY give out 75 to 100 mW. (See
cautions, above. --- Sam.)

Other things of interest is the 1,064 nm IR was negligible in power, only about
0.03 mW and IS NOT BLOCKED BY THE LITTLE BLUE FILTER. When at 85 to 90 °F
pump diode leakage was negligible also, but if it's cold, say 55 °F pump
leakage was over 50 mW but this IS blocked by the filter. It is also blocked
by the filter in my power meter too so I had to remove it to take a reading.
(The power meter probably also reads load at 1,064 nm. --- Sam.)

Despite the high power, this is not quite as much of a hazard as this was right
at the output of the brass part, by the time it reaches the output lens it is
reduced to only 7 mW or so and diverges very fast. The YAG beam is concentric
with the green beam.

The laser's life as a pointer is over, but it is turned into a nice module. I
replaced the cheap lens in it with a nice 1/2" diameter lens assembly from a
target designator. The assembly also gives it the badly needed heat sinking the
module calls for. The best part is though the beam is now about 1/4" diameter it
has SERIOUS range and can go 25 feet and still be about the same size!

This probably only makes any sense for power hungry green laser pointers since
the batteries in red ones should last a long time due to their lower current
drain (about 1/5th to 1/10th that of greens).

The problem with using NiCd or NiMH cells to replace Alkaline types is that
since the voltage is lower (1.2 V/cell versus 1.5 V/cell when fresh), the
output may not be as bright if the pointer doesn't include decent regulation
or its compliance range is inadequate. Thus, it will be necessary to adjust
or change whatever is used for current control in your pointer so it provides
the proper current to the laser diode at the lower operating voltage of the
rechargeable batteries. Note, however, that since the A-hr capacity of
rechargeables is less than that of Alkalines, lasing time will be reduced if
they are used. (This is somewhat compensated by the flatter discharge curve
of NiCds and NiMH cells and your mileage may vary.) Of course, you risk
blowing the circuitry and/or laser diode should you then install Alkalines,
so you may not be able to easily go back to them. As with the other comments
on modifications to laser pointers, this is quite risky both in terms of
possible damage to the laser diode as well as being able to make any
modifications to the teeny tiny circuit board if needed.

I've have heard of people (apparently with money to burn), successfully doing
this with a green ($$$) laser pointer. They changed the value of the resistor
used to set the laser diode current and were able to get slightly more power
at the same time (expected life unknown). (Interestingly, at the original
power, the beam was TEM00; with increased power, it became multimode.)

For a red laser pointer which already has an internal driver circuit (not just
a resistor), replacing the batteries with a regulated DC power supply
having the same voltage as the batteries should work. Or, simply using
external D cells instead of the internal AA or AAA or watch batteries
will work wonders for on-time. If there is already a driver inside the laser
pointer, the quality of the DC power isn't that critical but don't use
an unregulated wall adapter since its output voltage may be double or more
of the listed value when lightly loaded and it may also have a lot of ripple.
But one that is properly regulated should be fine. If in doubt, measure the
output voltage of the candidate adapter. It should be very close to the
nameplate value if regulated.

This should also work for green pointers since their drivers tend to be
of decent quality. However, with the higher current they use, thermal issues
become important and running some for more than a few seconds or minutes may
result in overheating and if not damage, at least a reduction in output power
and/or wild power fluctuations. Of course, given the higher cost of green
pointers, there is more risk involved in any case.

For the really cheap red laser pointers with no regulator, an external DC
power supply can also be used but make sure it doesn't do nasty things
like spike or reverse polarity on power cycling. And, regulation is even
more important.

One caution is that there may be cases where the internal resistance of
the intended batteries provide part of the regulation. This is unlikely
to be an issue with red laser pointers using AAA or AA cells. But with
watch batteries, it's possible.

While the typical 5 mW laser diode may have a specified life in excess of
100,000 hours (8 years, yeah, sure!), one often finds that the $6.95 variety
of laser pointers last a whole lot less than 8 years. :) It isn't possible to
provide a universal procedure that will get the most life from any laser
pointer. However, knowing that excessive current and singular overcurrent
events ruin laser diodes should provide a basis for some recommendations:

Run at less than maximum output. With a really cheapo design, this can be
done by adding a small resistor in series with the batteries (a few ohms).
Just enough to result in a noticeable decrease in brightnness. Yes, I know,
you selected the thing based on the absolutely highest intensity you could
find and want every photon of it!). For those with regulators, this may still
be beneficial since it will reduce the stress on the regulator components.

Minimize on/off cycles. If you are running with an external power supply,
consider leaving it on rather than pulsing it everytime you want a beam.
Even with batteries, some compromise between battery life and laser pointer
life may be worthwhile.

Redesign the regulator or add your own. :) This is left as an exercise for
student!

You may be better off buying a better quality diode laser module as they will
have the necessary current regulator using optical feedback and other laser
diode protection circuitry. While diode laser modules are generally much more
expensive than cheap laser pointers, there are some that are cheaper than
fancy laser pointers (which still may be low quality inside). Got that? :-)

For applications like communications, and laser shows, the output of the
laser must be able to be modulated, possibly at a high frequency. Where
a system is being designed from scratch, this capability is straightforward,
though not necessarily easy, to include. However, what about modulating
an existing system? The answer really depends on how it was designed.

Cheap laser pointers with no current regulator: Ironically, these
may be easiest to modulate. The Far East imports generally don't even include
any filter capacitors so applying a suitable signal as the power input should
permit very high speed modulation. The driver must be able to supply
the required operating current and be protected agains overshoot but beyond
that, the laser diode will ultimately limit the bandwidth.

Laser pointers or diode laser modules with regulator: These
generally include some filtering on the power input, possibly large enough
to limit modulation via switching power on and off to a few Hz. It may be
possible to remove, or reduce the size of the filter capacitors, but this
must be done with care since the laser diode protection may depend on the
value of these relative to the capacitance of the laser diode or attached
to the laser diode. The result may be a very quickly dead laser.

Another technique which is best for high speed modulation is to add a
direct input via a resistor (both directions) or resistor and diode (one
direction) to add or subtract current from the laser diode. The optical
feedback wlll attempt to maintain a constant output power but since that
is generally heavily filtered, will not respond fast modulation. Of course,
care must be taken to assure that the maximum current can never exceed
the rated value for the laser diode.

(From: Tom Becker.)

"I've been using a technique that has worked without failure for a long
time, on both a 5 mW 635 nm module and a 40 mW 780nm module with an
appropriate resistor change. It simply steals laser diode current,
allowing the automatic power control to function normally. This permits
modulation at megahertz rates. I've used it to carry a 10 MHz serial
data-stream. See Tom Becker's Diode Laser Module
Modulation Modification. The original you'll recognize as very common -
with my modifications asterisked."

Laser diode modules with modulation input There are two types of
modulation that may be supported: Analog and digital.

With analog modulation, the output intensity should be related to input
voltage between limits. For example, a 0 to 1 V signal may result in an
output between that varies between 1 and 10 mW. The limits may be
adjustable (gain and offset). A maximum modulation frequency may be
specified but unless the datasheet explicitly lists what this means, it
could be at the 3dB point, some other reduction in output amplitude, where
the output can no longer swing fully on and fully off, or something else.
Where the input is AC coupled, there will also be a lower limit greater
than DC/CW.

With digital modulation (often specified for TTL input levels, 0 V for
a logic 0 and +3 to +5 V for a logic 1), the output can be controlled
between two intensities. These may be adjustable. What happens in between
the logic 0 and 1 input levels is undefined. There will usually be a
maximum modulation rate specified which may be a frequency or a minimum
high/low pulse duration.

There is no standard for either type of modulation. The only way to know
for sure what the specs really mean is to contact the manufacturer and hope
they know what the specs really mean. :)

Science Toys has some suggestions
for doing this on their
Light and
Optics Page hoping you'll buy the components from them. But there's
a good chance what's in your junk drawer will work just fine with a Dollar
Store laser pointer.

It's a cute idea: Introduce an external light source to 'fool' the internal
optical feedback circuit into thinking that the laser power is higher than
it should be. The driver will then cut back on current to compensate.
If you shine certain laser pointers at a mirror, their output will drop
dramatically. However, this effect may be due to the monitor photodiode
sensing the added light and cutting back on laser diode current, or
due to light getting inside the laser diode cavity and messing up lasing.
Apparently, the latter effect as unlikely as it sounds, may be the one that
is more likely, at least with certain models.

One way to tell which effect is causing the change in output power is to
measure the laser diode current: If it drops with the reflection, the cause
is likely the simple optical feedback mechanism. If on the other hand it
increases, then laser instability is likely. Also see the section:
Causes of Laser Pointer Output Power Changing
When Directed at a Mirror.

Even if the photodiode sensitivity is the cause, several factors conspire
against this being a viable technique in general (though it may work with
specific devices):

The monitor photodiode (PD) is deliberately made quite insensitive since
it 'sees' about the same amount of light as is produced at the laser diode's
output but from the rear facet. So, it's bathed in light which is quite
strong compared to anything that could be introduced externally unless it is
another laser.

Hitting the PD would also be quite a challenge. Although it is relatively
large at least in comparison to the laser diode (LD) chip, it sits behind it
and may be partially blocked. So, any external source would have to
adjusted to focus on the PD after passing though the collimating lens
of the laser pointer or diode laser module.

The laser diode drive circuitry likely includes some amount of filtering
using large capacitors. This would seriously limit the frequency response.

And, if it is actually a lasing interference effect, good luck succeeding in
getting anything to be repeatable or stable unless you have a granite block
or sand-box holography setup. :)

If you still insist on experimenting, be aware that while this appears to be
safe for the laser diode, there is no way of knowing for sure without tests.
There could be funny resonances in the driver that will blow your laser diode
at certain frequencies! And, if the effect is due to lasing instability,
the regulator may attempt to boost the current to compensate resulting in
possible overheating of the laser diode, driver, or both.

My informal experiments have turned up both effects, one of each for a couple
of laser pointers and quite noticeable photodiode based power suppression with
an NVG D660-5 (just happened to be one I tried) on an optical feedback
regulated driver - shining a laser pointer into the laser diode window resulted
in almost total supression of lasing. I suspect that the pointer affected
by interference inside the cavity went into overcurrent or thermal shutdown
(as it refused to lase at all for several seconds after the test). And, a few
days later, it was obvious that the output power had decreased and the beam
pattern was messed up, a sure indication of facet damage, which probably
happened immediately but I just didn't notice it.

The following discussion resulted from the claim (mine and others) that
reflecting the output of a laser pointer or diode laser module from a mirror
might result in a decrease in output if it had optical feedback for power
regulation. On one laser pointer I have, there is absolutely no effect. On
another, output power drops by at least 50 percent. My assumption was that
it was the light reflected back and falling on the monitor photodiode that
caused the effect and not some weird interference to the lasing process. But
given what is described below, I'll concede that in many cases, it may indeed
be the latter.

It does seem that relatively low reflected power back to the laser diode can
affect lasing. This has been used to advantage in narrowing the line width
of common laser diodes with an external cavity. See, for example,
U.S. Patent #4,907,237: Optical Feedback Locking of Semiconductor
Lasers.

One way to tell which effect is causing the change in output power is to
measure the laser diode current: If it drops with the reflection, the cause
is likely the simple optical feedback mechanism. If on the other hand it
increases, then laser instability is likely.

However, suppose the returning beam hits the monitor photodiode. Since the
outgoing and return beams are mutually coherent, interference fringes will be
formed on the surface of the photodiode. If they are large enough as
they would be with very good alignment of the outgoing and return beams, and a
minima were to dominate the surface area, the feedback circuit would think
that the power was too low and increase current - possibly to destructive
levels.

Another possibility is that the return beam from the mirror precisely hits
the output facet of the laser diode. While this is a very small area, it
only needs to happen for an instant. The result is an extended cavity
which suddenly has a much lower loss due to the higher reflectance of the
external mirror compared to the cleaved facet. The result is a virtually
instantaneous increase in intracavity power and if the laser was running close
to the COD (Catastrophic Optical Damage) limit, poof goes the laser diode.
This would be more likely with a constant current driver but even in constant
power mode, the increase in intracavity power would take place in less than
1 nanosecond - much less than the response time of the feedback circuit.

One variable that can be played with in any experiments of this type is
the divergence of the beam: A collimated beam will be much more likely
to result in interference or instability effects as it will be returned
with virtually the same wavefront.

Adding a polarizer or polarizing beamsplitter aligned with the diode
polarization followed by a quarter wave plate would suppress most
back reflections. A very expensive optical isolator would eliminate
them almost entirely.

CAUTION: I have both first hand experience of damage to a laser pointer
diode and have also heard of diode failure from others
that may have resulted from these sorts of experiments.
A very nice laser pointer I have never quite recovered after seeing
its reflection and is now operating at about 1/4 power with very
noticeable facet damage. Others have reported instantaneous
damage to single mode (TEM00) laser diodes from reflections
having eliminated other possible causes. High power (e.g., 35 mW
and above) seem particularly vulnerable.

(From: John, K3PGP (k3pgpalltel.net).)

This is pretty much my findings here also.

However, since laser pens seem to be built as cheaply as possible there are
NO standards! What works with one may not work with another. This has
caused me untold grief when trying to discuss most anything about laser
pens!

I have a few laser pens here that go nuts when you aim them at a mirror.
With some pointers the mirror has to be precisely aligned much the same as
the mirrors at the ends of the laser cavity itself. With others the
alignment isn't as critical. These same pens seem to be unaffected by other
light sources shining back into the laser including light from another laser
pen with the same approximate wavelength.

I think the important fact to those those units that were affected is
whether or not the incoming radiation was precisely the same frequency as
the oscillation in the laser cavity. When this experiment is set up with a
pen that is sensitive to this effect, EVERYTHING affects the setup, even the
slightest vibration which makes sense (to me anyway!). It kind of reminds
me of the Michelson Interferometer or a holographic setup. I assume this
interference effect is the same effect noticed with many HeNe lasers where
no power sensing diode is involved.

(From: Sam.)

That would seem to confirm the hypothesis that interference with the lasing
process is taking place, at least for those cases. I'm surprised they would
be so sensitive.

(From: John.)

These pens seem to be somewhat rare though as most of the laser pens that I
have don't seem to care what you shine back at them. Since laser pens
differ so widely from one manufacturer to the next and even between
identical model numbers from the same manufacturer I'm not sure if the
differences are being caused by the use of different laser diodes or perhaps
this effect is somewhat critical as to the amount of current passing through
the laser diode or something else?

(From: Sam.)

Conceivably, the sensitive laser diodes are being operated on the verge of
mode hopping or something like that but I'm more inclined to believe it is
just a sample to sample variation or laser diode model dependent.

(From: John.)

When trying this experiment with several different HeNe lasers I've also
noticed that some are effected to a much larger extent than others. I'm not
sure why this is. Maybe it has something to due with the gas mixture, the
pressure, the current passing through the tube, or what else?

(From: Sam.)

Also mirror reflectivity and curvature. The gas mixture, pressure, and
current are probably less of an issue as long as it is running somewhere
around the correct conditions.

When you reflect a beam back into a HeNe laser, it's only .5 to 2 percent of
the strength of the output beam and order of .01 percent of the strength of
the circulating photon flux inside the tube unless the external mirror
is very close to being parallel to the output mirror. Then, there will be
multiple bounces and much of the light makes it back to the cavity... Hmmm.
The distance also matters due to interference effects and the curvature of the
mirrors affect the shape of the wavefront. Possibly HeNe lasers with close to
planar mirrors are more sensitive to this. However, just the light bouncing
back and forth and interfering with itself outside the cavity can
confuse the observations. What a mess. :)

In the old days before the dinosaurs roamed the Earth and even before cell
phones, laser pointers may have been constructed in a such a way that they
could be taken apart and put back together again. Regrettably, that is no
longer the case. Among your options are a hacksaw, lathe, hammer, and
Dynamite - or something stronger. :) It can be done but don't expect to get
the pieces together again, at least not in an aesthetic package.

For red laser pointers, note that some/many/most of the newest and cheapest
imports may not even use a packaged laser diode - the bare chip is attached
directly to a metal header next to the lens. I wouldn't be too optimistic
about repair or reuse of one of those.

The deconstruction process for a typical green (DPSSFD) laser
pointer - a much more complex device than the red variety - is shown in the
Laser Equipment Gallery (Version 1.47
or higher) under "Dissection of Green Laser Pointer".

I couldn't resist picking up 23 supposedly dead key chain-style laser pointers
on eBay. These were supposedly "dealer returns" which could mean anything
from the buyer didn't know how to insert the batteries to they were used for
1,000 presentations and then taken back to the store with a claim of being
defective (yes a few were obviously well worn). :) They are the type that
come with zillions of pattern heads (actually 5 to 12 including the only
one that is useful - the clear one), just use a resistor to limit current
to the laser diode (no driver) from the 3 watch-style button cells, and
were all made in China, probably by the same manufacturer. While there were
minor variations in case styles, they were all similar in construction to
Components of Simplest Red Laser Pointer internally.

Despite their simplicity, the power and beam quality are generally comparable
to the older more complex red laser pointers, though the overall manufacturing
quality and consistency leaves something to be desired (see below) but what do
you want for a couple of dollars?

Note that a defective or damaged laser diode were no more likely than anything
else (and one of these would actually lase but only with 4 cells instead of 3).
All the others with actual problems could be repaired easily except
for those that were intermittent which would require extracting the guts from
the case. The problem in the one sample I disassembled was bad contact
in the press-fit connection between the cast metal lens housing and copper
of the circuit board on which the bare laser diode chip was mounted.
The beam focus on all the pointers was decent. Power on all except
the weak or dead ones was probably between 1 and 3 mW (I didn't measure it).
About 2/3rds of the batteries were in new or close to new condition,
charge-wise. A large precentage of the bad ones were bulging and a couple
had non-explosively disassembled themselves, likely due to a short circuit
as a result of the defective or missing battery insulators.

One nice characteristic of these pointers is that their output power can be
varied smoothly either by using a variable external power supply or by adding
a pot in series with the batteries or power supply. Just make sure the
power source - be it a wall adapter or lab supply - is well behaved and
can't overshoot or be accidentally set much above the approximately 4.5 V of
fresh batteries. At 4.5 V in, a 100 ohm pot will vary the output power from
below lasing threshold to maximum. The beam was still decent below lasing
threshold (from LED emission) and would be acceptable for applications not
requiring the narrow line width and better coherence of a true laser.

Suppose someone offers you a diode laser module that has been damaged by
applying incorrect power (the smoke all leaked out) for $5. Should you accept
it? Is there any hope that the laser diode itself survived?

The quick answer is a definite maybe IFF the module or pointer can be opened
for examination or repair. If it is a potted block, forget it.

The chances of success are much greater for a diode laser module since it is
likely to have a proper laser diode driver with current regulation and optical
feedback. These are typically so over-designed that while applying excessive
voltage (well, within reason, not 120 VAC to a 5 VDC module!) or incorrect
polarity may blow some components, chances are that the laser diode itself
won't feel a thing and will survive unharmed.

Assuming you can get inside, repair should be possible. And, even if you end
up having to replace a 5 mW laser diode (for, perhaps $10), you have made out
well. High quality diode laser modules go for anywhere from $50 to $300.

However, depending on design, a laser pointer could be totally destroyed by
even modest overvoltage (say 5 V instead of 3 V from 2 AAA batteries) or
reverse polarity. Some of these don't have anything more than a resistor
for current limiting. So the laser diode could very well have been damaged
or turned into a DELD (Dark Emitting Laser Diode) or expensive LED. All you
may end up with is a nice (or not so nice) case. :-( Of course that in itself
may come in handy to package your own laser diode and driver - ignoring what
was originally there. However, see the next section for more on this exciting
topic. :)

The following applies to laser pointers containing just a battery, driver,
laser diode, and optics. For now, this is only the red variety though
pointers using the Nichia violet laser diode, as useless and expensive
as they may be, would also qualify. :) For green or other DPSS laser based
laser pointers, there is the additional complexity of the DPSS laser module
itself. See the section: Repair of DPSS Laser
Pointers. And, for older style helium-neon laser based laser
pointers, see the chapter: HeNe Laser Testing,
Adjustment, Repair.

With prices as low as $2.00, serious troubleshooting and repair of a cheap red
laser pointer probably isn't worth the effort, time, and expense. But if you
have one with 58 pattern generating heads or just want the educational
experience, there may be a possibility of repair even though many of these
things are not designed with user serviceable parts inside.

Refer to Typical Red Laser Pointer for a general
idea of what to expect. The detailed disassembly procedure will depend on
the exact model. A combination of screw, press-fit, and glued construction
is likely. Non-destructive disassembly may not be possible for some.

Focus adjustment: Where there is no external focus adjustment, it
may be possible to remove the front bezel and then access an internal focus
ring. With luck, it can be turned with a flat blade screwdriver or other
suitable tool to tweak for best focus. Take care not to scratch the soft
plastic lens. However, it may be necessary to remove a bit of glue locking
it in place. If the entire lens assembly is glued - no threaded barrel -
adjustment may not be possible.

Water damage: If the pointer got seriously wet, immediately
remove the batteries and get as much liquid out as possible. If the liquid
was just plain water, waiting long enough for it to completely dry (perhaps
with modest assistance from a heat gun or blow dryer on the low setting)
before replacing the batteries may be all it needs if nothing got on the
inside of the optics. However, if the liquid was something other than plain
water, particularly a corrosive substance like salt water, complete
disassembly and cleaning will be required. Where the laser diode is in a
hermetically sealed (usually 5.6 mm) package, quick action should permit the
pointer to be salvaged. However, with some newer really cheapo units using
bare laser diodes, any contamination that reaches the laser diode chip may
be bad news indeed. In the latter case, very careful cleaning with pure
alcohol or acetone may save it but this has to be done before attempting
to power the diode - anything on the facet while powered may be terminal.

Physical damage: Where parts are actually broken, replacement may
be the only option. For the collimating lens, something from a CD player
may actually be good enough. However, suitable lenses can now be obtained
from many laser and electronics distributors.

Here are possible problem areas for a pointer that is weak or dead and
hasn't been run over by a Sherman Tank:

Battery: Test under load or replace. Confirm that the cells are
correctly oriented. For most laser pointers, the positive points away
from the driver, opposite of a flashlight! Check for dirty or corroded
contacts and clean if necessary.

Switch: Using an ohmmeter, check for reliable operation of the
push button switch. Replacements may be obtained from a service parts
supplier (they are often similar to those used on VCRs and other consumer
electronic equipment).

Laser diode driver: Look for cracked solder connections,
particularly at the switch and where the laser diode attaches to the
driver board - these get abused. If there is a power adjust pot, mark
its exact location and then turn it back and forth a couple times to
clean its track. Test any diodes and transistors for shorts and opens.
If there is an IC and it has a part number that can be identified, search out
the datasheet.

Laser diode: See the sections starting with:
Determining Characteristics and Testing of
Laser Diodes to determine if the laser diode is still good. It may
be possible to replace a bad laser diode but the operating current and
monitor photodiode (if used) current specs would need to match fairly
closely. This could be an opportunity to improve the visibility by using
a shorter wavelength laser diode like 650 nm instead of 670 nm. However,
avoid the temptation to increase the output power above the original 5 mW
rating. Not only are higher power diodes much more expensive and just as
easily blown, 5 mW is the legal maximum (I had to say it).

There are at least 3 surfaces that can collect dirt - the two sides of the
lens (it is probably a single element) and the exterior of the laser diode
window. However, in all likelihood, only the exposed surface of the lens will
need cleaning.

First, gently blow out any dust or dirt which may have collected inside the
lens assembly. A photographic type of air bulb is fine but be extremely
careful using any kind of compressed air source. Next, clean the lens itself.
It may be made of plastic, so don't use strong solvents. There are special
cleaners, but isopropyl alcohol usually is all that is needed. 91% medicinal
should be fine, pure isopropyl is better. Avoid rubbing alcohol especially if
it contains any additives.

Lens tissue is best, Q-tips (cotton swabs) will work. They should be wet but
not dripping. Be gentle - the plastic (probably) or glass and particularly
the anti-reflection coating on lens is soft. Wipe in one direction only - do
not rub. Also, do not dip the tissue or swab back into the bottle of alcohol
after cleaning the optics as this may contaminate it.

The alcohol should be all you need in most cases but some types of dirt (e.g.,
sugar) will respond better to just plain water.

The inside surface of the lens, any other optics, and the window of the laser
diode can be cleaned in a similar manner should this be necessary. Usually,
it is not.

Do NOT use strong solvents (which may attack plastic lenses) or anything with
abrasives - you will destroy the optics surfaces.

CAUTION: Lenses or other optical components may be bonded or mounted using
adhesives that are soluble in alcohol or acetone (but probably not water).
Don't make the mistake I made and use too much solvent. I still have not
found the tiny collimating lens that popped out of a laser diode module and
is now likely lost forever to the basement floor. Crunch :-(.

Even a 1 mW laser beam can potentially produce permanent damage to the CCD
or silicon sensor array insid
e a video or still digital camera.

If the camera is focused at infinity, a collimated laser beam will be focused
to a tiny spot on the image sensor. Whether damage will occur depends on
many factors including the type of image sensor, quality and focus of the
optics, and how long the beam is held in one place. A 1 mW beam (much less
than what some laser pointers produce) is roughly equivalent to the brightness
of the noonday Sun at the equator on a clear day and when focused to a 10 um
spot (the approximate size of one pixel on a typical video camera) it becomes
10,000 times more intense! Needless to say, pointing a camera at the Sun
is generally not recommended.

Anatomy of Fiber-Coupled Laser Diodes

Fiber-coupled laser diodes or diode lasers - same thing - aren't the sort
of thing you will find at your local K-Mart but
may turn up surplus from communications, medical, or other applications
requiring delivery of a high power laser beam over a fiber optic cable.

WARNING: Class IV laser products - the output from the fiber will destroy
vision and set things on fire!

CAUTION: When using fiber-coupled laser diodes (or any high power fiber-optic
system), the cleanliness of the fiber ends is critical. Any speck of dirt
or contamination will be burnt to a crisp by the high optical power density.
In addition to the immediate power loss due to absorption and scatter, the
thermal effects may damage the fiber (requiring cleaving, remounting,
and repolishing). And back-reflections can actually damage the laser diode
shortening its life or resulting in a permanent power loss and/or instability.

Fiber-coupled laser diodes are much easier to use than bare laser diodes even
though they still need an external high current driver. (Of course, they are
also much more expensive.) Aside from the physical protection provided by the
packaging, the output of the fiber is a nice circular beam with modest
divergence (about 16 degrees full angle) which doesn't require correction for
astigmatism or asymmetry. Thus, simple lenses can be used for collimation and
focusing. I've used a good sample of the 808 nm version of the first laser
described below to pump the guts from a green (DPSS) laser pointer just by
holding the end of the fiber next to the Nd:YVO4 crystal. After
adding a coupling with a GRIN lens for focusing, I can get a few mW of green
light from it though I suspect the diameter of the pump beam is still larger
than optimal. These will also easily pump the CASIX DPM0101 and DPM0102
Nd:YVO4/KTP composite crystals as well as other microchip lasers.

A typical unit is shown in Typical Presstek Fiber
Coupled Laser Diode along with a fiber focuser/collimator. This model
was probably actually manufacturered by Opto Power
and will thus have similar internal construction to the one described below.
However, these and similar laser diodes from graphic arts platesettings and
similar equipment generally operate at between 820 and 880 nm which is NOT a
useful wavelength range for DPSS laser pumping. So, just because it walks and
talks like a fiber-coupled laser diode does not mean it will of value other
than as a burning laser. :( :) Typical characteristics of platesetter diodes
can be found in the section:

(Note that Opto Power is now part of Spectra-Physics but these lasers predate
the merger which may be one reason for the very different types of technology
used in the construction of the first three lasers, below).

WARNING: The output beam of high power laser diodes with an attached microlens
(or other collimating optics) is much better collimated than we are used to for
laser diodes - closer to that of a "real" laser. The divergence (total at
the half power point) is typically 10x4 degrees as opposed to 10x40 degrees
for a bare laser diode. What this means is that both the direct beam and
any specular reflections are MUCH more dangerous to vision even several
feet away from the source. Even the reflection from a shiny IR detector
card can be dangerous. This is especially scary for people who have become
complacent working with laser diodes being used to beams that spread out
to safe levels in a few inches.

The overall package is 1.5"(L) x 0.75"(W) x 0.5"(H) and is made of a block of
gold plated brass with a milled cavity. There are red and black wires for
power and a single-mode fiber with SMA 905 connector for beam delivery.

After prying off the Epoxied lid, the following can be seen:

The laser diode is an open heatsink device similar or identical to the
Spectra-Physics (now Newport) Prolite SCT open heatsink semiconductor
laser. (Go to Newport and search
for "Prolite SCT". It includes reverse polarity
protection in the form of a second laser diode chip wired in parallel with
the primary laser diode. (It may actually lase if driven with enough current
but since it's mounted upside-down and there is only one bonding wire, it
would probably fail at a relatively low drive current. The beam also isn't
anywhere useful.) The heatsink is fastened to the case with a 2-56 cap
screw. In between is a metal thermal washer (probably indium, no silicone).

Multiple gold bond wires connect the laser diode to a ceramic circuit
board just for connections to the external power supply wires (there are
no components on the circuit board).

The fiber cable enters a hole on the side and is securely fastened with
Epoxy. At the point where it enters the interior of the package, the
central fiber is entirely naked. :)

The fiber end is located precisely in front of and nrealy against the
output facet of the laser diode by a blob of Epoxy on a bit of ceramic midway
between the wall of the package and the laser diode. The length of the output
facet and size of the fiber core are similar - order of 100 um. Therefore,
the fiber end must have been adjusted for maximum output coupling using an XYZ
micropositioner (there is evidence of a little mark where it would have
been attached). Then the Epoxy would have been UV cured to lock the fiber in
place. Interestingly, the fiber isn't supported beyond the Epoxy - about 1 mm
sticks out. So, I imagine that tapping on the unit while operating would
cause the laser output to be modulated at least somewhat.

There is a cylindrical microlens glued to the edge of the laser diode
heat sink to reduce the vertical divergence and gain more efficient coupling
to the fiber. It's hard to make out in the photo and not obvious even under
a microscope but the optional use of such a microlens to reduce vertical
divergence to about 6 degrees is mentioned in Opto-Power's description of
their unpackaged laser diodes.

Here are a couple of other lasers that yielded to my set of hex wrenches - no
chisels or cutting torches required. They were mostly dead prior to surgery
so no need to call out the SPCL (Society for the Prevention of Cruelty to
Lasers!

These two are strange. They have a rated output power of 3 W into a
multimode fiber. Input voltage is the usual 2 V but the operating current
is supposed to be about 10 A (when new) with a recommended current limit
on the driver of 20 A!!!. They only differ in wavelength.

Both had problems with low output power after relatively minimal use -
probably a few dozen hours at most. Much of it could be restored by
readjustment of the internal alignment - which is surprising for a
packaged laser diode. However, as you will see, these aren't ordinary
diode lasers! But at least almost everything is adjustable, if I only
knew the proper procedure

The model number of the first one is OPC-D003-814-HB/100. Its spec'd
wavelength is 814 nm special ordered to pump Nd:Mg:LiNbO3
(Neodymium doped magnesium doped lithium niobate, which incidentally lases
at 1,084 nm.) However, when running at low power or with suitable cooling,
will operate at 808 nm. The package is large - about 15 cm in length. See
Opto Power High Power Fiber-Coupled Laser Diode -
Overall View. A closeup of part of the interior is shown in
Partial Interior View of Opto Power High Power Fiber
Coupled Laser Diode. (Removing the rest of the case is possible but
more work than I could justify just to show the really boring output optics!)
The description below applies to both models:

The emitter is probably a small laser diode bar, about 5 mm in width. The
product blurb calls it a "diode laser array".

The diode's output goes through a cylindrical microlens to collimate the
vertical (fast) direction.

Then it goes to a glass strip I can't really identify - it looks to be a
wavy structure with about a 1 mm period. The beam that exits this thing is
about 5 mm wide with a half dozen or more very distinct peaks in intensity
(but only if the diode is actually lasing - when below threshold
and just acting as an LED, the intensity profile is quite smooth). On
other high power diode lasers, a similar wavy thing is called an
"integrator" or "homogenizer" and serves to make the beam profile
more uniform. Here it probably means that they don't attempt to line
up each of the emitters with an entry point on the mirror (see below)
but want the beam uniform.

The beam then hits a thin glass plate with mirror coatings over very
specific areas on both sides positioned at around a 45 degree angle in both
X and Y which obviously can be very precisely adjusted and then clamped down.
It appears as though the horizontal beam hits a tranparent area of
the coating on the front surface and then sub-beams are channeled by multiple
reflections and lined up vertically into a single beam before going to the
subsequent optics. This must be the magic Opto Power had been touting
when they marketed these lasers. :) In essense, it breaks up the diode's
output into a series of spots that are single mode vertically and multimode
horizontally, and stacks them vertically so the slow (multimode) axis is
narrowed at the expense of the fast (single mode) axis. The result would be
similar to taking a series of smaller multimode diodes (e.g., 1 W, 100 um
stripe) and arranging their emitting apertures in a vertical stack, but
at lower cost.

Then on to a short focal length cylindrical lens (oriented vertically)
followed by a longer focal length cylindrical lens and a short focal length
spherical lens to focus it into the fiber core. This is conventional beam
shaping.

A temperature sensor is included but no TEC. In the original system, the
fiber-coupled laser diode was mounted on a massive TEC which was on a massive
forced-air cooled heatsink. Given the maximum power dissipation, water
cooling might be preferred.

This particular unit originally had no output and might have been
dropped as the final focusing lens has slipped vertically in its
set-screw locked mount. Fixing that was easy, but someone (I won't
name names!) had attempted to adjust the angular plate before realizing
the lens was out of position. So far, I have been able to get
what would be around 2.24 W at 10 A (only tested to 4 A) into a
100 um core multimode fiber (which is what's called for in the spec)
though the diode inside should be capable of around 5.5 W
at 10 A (based on my measurements to 3 A). This represents
about 40 percent of the output of the diode making it into the fiber.
With the original 100 um core fiber that came with the laser, the performance
is really dreadful - I suspect that particular fiber is damaged. With a wide
(500 um) core fiber, most of the light available at the output of the focusing
lens does make it into the fiber. This suggests that the problem may be not
so much in getting light to the output optics, but shaping the beam in such
a way that most of it can be coupled into the 100 um core fiber. I have
carefully adjusted the fiber mount in X, Y, and Z, so that should be close
to optimal. The magic angled plate may still be seriously misadjusted (but
I doubt it) or damaged, and the focusing lens may be a bit out of position
though I doubt that's the cause. The diode may be weak - it did have
a run in with our "killer driver" - one that tended to zap laser diodes at
random due to overcurrent (though it's hard to comprehend how even that
unit could damage a diode perfectly happy with 20 A!). The slope efficiency
is 0.68 which is somewhat low this type of diode but that could be due to
losses from the (non-AR coated) microlens and rippled plate.

The only values that were actually measured were the bare diode at 2 A and
to determine threshold, and the fiber outputs up to 3 A. The others were
estimated. That's why some of the numbers seem so perfect! My LaserCheck
already had enough burnt spots in its plastic case. :)

Fine tuning the alignment (including those optics I haven't yet touched!)
might restore the missing power but I doubt that's really possible in finite
time while remaining sane without the original factory jigs and setup
procedure. Or justifiable given that I currently don't have a good use
for this beast. Devices like the much smaller, simpler, more efficient
Opto Power fiber-coupled laser diode described above are perfectly adequate
for an output power up to 1 or 2 W. Of course, this one should still
produce full power at way below the recommended 20 A current limit so
perhaps I shouldn't be complaining very much. A 3 W fiber-coupled laser
with a 100 um core fiber is rather impressive. The original price was
also rather impressive - just under $6,000! :)

At least I was able to use the 100 um fiber to pump a CASIX DPM0102 green DPSS
composite crystal and get some green light! However, at around 3 A and 35
pounds (including driver), this would have to be the biggest most inefficient
laser pointer on the face of the Earth! :)

The other unit has a model number of OPC-D003-980-HB/100. Its wavelength
is 980 nm which is used to pump erbium doped materials that
lase in the area of 1550 nm (actually over a range of more than 50 nm).
Much of this diode's output makes it though the optics but less gets
into the fiber. The threshold is much lower as well though the slope
efficiency isn't very good. Realignment of the angled plate and fiber
connector was required on this one as well even to get to this point.
Originally, there was very nearly exactly 0.00 mW making it to the fiber
but no evidence of trauma:

As above, the only values that were actually measured were the bare diode
at 2 A and to determine threshold, and the fiber outputs at 2 A. The others
were estimated.

My conclusions from examining and aligning these lasers is that while the
design is clever, it's way to finicky. Both of these lasers had seen
relatively little use in a university lab environment. While one had
probably been dropped knocking the focusing lens out of position, it may
have already been weak when that happened. Possibly just repeated thermal
cycles resulted in various optics like the angled plate walking away
from proper alignment. None of the adjustable internal optics had any
adhesive to lock their position, generally common in other lasers.

Both of these specimens probably date from the mid 1990s.
Nowadays (2008), companies offer micro-optics to do the same thing
with much higher efficiency that are both considerably smaller, are easier
to align, and are more robust. One example is the
LIMO
Beam Transformation System (BTS-150/500D) and Hybrid Optical Chip (HOC)
for coupling of laser diode bars with 19 emitters spaced 500 um apart, into
a multimode fiber, with an efficiency of 70 percent for a 200 um core
diameter.

This is an 803 nm unit with a power output of around 1 W model unidentified.
It's application is also not known. Correction optics consist of a short
focal length collimating lens glued to the rectangular diode "H" package to
collimate one axis, a cylindrical lens to correct the other axis, and an
adjustable (in X,Y,Z) focusing lens to get the light into the fiber core.
The distance from the focusing lens to the fiber tip (Z) is quite critical
but the position of the fiber in X and Y has a broad peak since the beam into
it is quite well collimated and smaller than the lens.

Unlike the Opto Power unit above, this Spectra-Physics "FCBar" places
a special 19 core fiber end in close proximity to a 1.5 cm laser diode bar.
See Spectra-Physics FCBar Fiber-Coupled Laser Diode
Bar - Overall View and Spectra-Physics FCBar With
Diode and Fiber Separated. The large black object is a relay which shorts
the laser diode terminals when no power is applied. There is also a
personality EEPROM on the PCB. There is a fiber microlens for fast-axis
collimation. Since the fiber cores are relatively large (probably around 200
um), high efficiency coupling can be achieved as long as they are relatively
close and aligned with the emitting apertures. Clamps and screws
allow the tip to be positioned precisely so each of the 19 cores aligns
with its mating aperture, close but not touching - about 0.1 mm
in the samples I've seen.

There is a temperature sensor but no TEC. The module was designed to
mount on a "cold plate" fed directly by a hermetic recirculating chiller,
water chiller, or tap water.

At the other end of the armored cable, the 19 fibers terminate in an FC
connector with a large multimode core. Why 19 fibers? Probably because 19
cylinders pack nicely into a nice hexagonal array with a somewhat circular
perimeter. The series is 1, 7, 19, 37, 61,.... Of course, other values
will work and for most applications it doesn't matter. The lower power
version of these modules use a 7 core fiber.

The laser diode bar has a threshold current of about 6 A and should be capable
of at least 15 watts of output from the fiber. It was part of a solid state
laser which was pumped by a pair of these FCBar modules. The output power
of the solid state laser at 1,064 nm was probably around 10 W. I plan to
test this diode further in the near future. Another unit I am testing
has a threshold of 12 A, with a maximum rated output of 26 W.
Its output at 25 A is 10 W with 22 W at about 40 A. Based on the
test data for a similar new diode, it's a bit weak - 26 W at 40 A is
typical. But it would probably still meet rated specifications.
The model number is DMJ-ZLM-24-08. It's called an FRU Diode Module.

A datasheet for the versions of these diodes in current production
(but without the electronics) would appear to be a version of the
Spectra-Physics (now Newport) Prolite SCT series.
(Go to Newport and search
for "Prolite SCT".) The exact models may not
be listed here as there may be versions with intermediate rated output power
(like the 26 W) not shown. But, it should be possible to interpolate
power and current to get a reasonably accurate idea of the behavior.

I've come across several fiber coupled laser diode array modules which
have a symptom of an almost dead short across the diode even with all
other components (relay, reverse protection diode) disconnected. Upon
disassembly, there was a very obvious carbonized area on the face of the
diode as well as carbonized crud on the face of the fiber tip. I have
preliminary results of repair attempts on two such modules. There is no
way to get full power as one or more of the individual diodes has basically
blown up. But some or most of the remaining ones may be salvageable.

I believe the cause of these failures is contamination or moisture getting
onto the front facet of the laser diode array. The modules that have failed
in this way are not hermetically sealed due to the passage of the thermistor
temperature sensor leads through oversize holes in the PCB. Three units
arrived in this shorted state. One unit failed while I was attempting to
cool it on one of those ice packs used for keeping your lunch cool and I
expect there was condensation.

Of course, if you have the big $$$ available, replacing the laser diode
assembly itself is likely to be much more useful than the kludge below.
Then, it would be a "simple" matter of realigning the fiber cable. But, the
diode will have to come with the fiber microlens for fast axis collimation
(added $$) and its individual emitting apertures must have the same spacing
(pitch) and similar size compared to the original. In cases where that
was a custom OEM part, a suitable replacement may not be available.

The following is not something you should admit to in the presence of
your boss, if he/she has anything to do with laser diodes. It's a long
shot but if the alternative is the trash, there is nothing to lose.
Here's the procedure. No guarantees of anything! Refer to
Spectra-Physics FCBar Fiber-Coupled Laser Diode
Bar - Overall View.

Detach the fiber-optic cable assembly by removing the two screws holding
it to the silver colored block of the FCBar module. Once the cable is
free, inspect the elongated "tip" for debris and damage. On each of the
four units I've seen, there was a very visible clump of carbonized debris
covering the fiber tip opposite the diode(s) that shorted. Lens tissue an
alcohol easily removed it without a trace. Once it has been cleaned, set
the cable aside with protection for both ends.

Detach the printed circuit board by removing the two Philips head
screws connecting it electrically to the laser diode, the 4 hex head
screws holding the PCB in place, and unsolder the two thermistor
wires (not on all units). Set the PCB assembly aside in a safe place.

Remove the hex head cap screws attaching the diode terminals to the
diode block and pull out the copper terminal assemblies. Set them
aside.

Remove the remaining 3 hex head cap screws holding the diode block
in place and set them aside.

Carefully loosen the diode block from the heatsink compound or indium
foil and remove it.

Closely examine the output area of the diode. Opposite where the crud
was on the fiber tip, there will be a corresponding blackened/melted area
behind the fiber microlens. The lens itself may also be damaged. Hopefully,
the short circuit is localized to this area.

The trick is to carefully scrape away the front facet of the bad diode
with a knife or razor blade to clear the short. What I suspect happens is
that a bit of contamination or moisture on the front facet creates a
conducting path. Current builds up in the immediate vicinity quickly
heating and destroying the diode. The failed diode and at least one
on either side will likely remain dead but hopefully, some, most, or
all of the others can be salvaged. If the failed diode is near the
middle of the array, it may be possible to do the scraping without
removing the fiber microlens. However, if it's near one end, at
least one end of the fiber microlens should be detached to provide
enough compliance to get the knife or razor blade behind it.

Work in small increments and use a current limited power supply to
check the short. At some point, the remaining shorting crud may
be vaporized and the diode will suddenly spring to life.

Once the short is removed, the module can be reassembled. If the
fiber microlens popped off or broke, you're on your own. If only one
end came loose, use a drop of Epoxy to reattach it but make sure excess
doesn't interfere with the location of the fiber tip.

Reinstall the diode assembly. Center it and reattach the electrical
terminals.

Carefully check that the fiber tip of the fiber-optic cable assembly
is about 0.1 mm from the fiber microlens when fully seated. Above all,
it must not touch as the fiber microlens will likely shatter in that case.
See the next section for the fiber replacement procedure.

I told you this was a long shot! Comments welcome but nothing like: "There
is no way in h*** that this can work!". :) I was able to recover 6 of 19
emitters on one module and 14 of 19 on another. Whether they will survive
for any length of time is another matter.

Replacement of a damaged fiber is possible without fancy jigs. Clean the
fiber tip to remove all traces of contamination. Remove the
PCB or cover on the FCBar module so that the distance to the fiber tip
can be set precisely without bashing the fiber microlens. Set it so
there is a just visible gap - about 0.1 mm. Then, with
the two holding screws not quite tight, drive the diode at just above
threshold and adjust the X and Y position for maximum coupling, then
tighten the screws. It shouldn't be possible to be off by an entire
emitter spacing and still get coupling. An IR viewer or IR camera
is desirable to monitor scattered light inside the diode package and
minimize this as well. CAUTION: DO NOT drive the diode at more than
minimal power until the alignment has been optimized as excessive
back reflections can damage it instantly. Note: The emitter spacing
(pitch) varies among models. The units described above have a pitch
of about 0.78 mm. Others may be 0.5 mm or 0.65 mm or something else.
Of course, the pitch must match exactly!

Low Power Visible and IR Laser Diodes

These are the typical 3 to 5 mW (maximum power) visible laser diodes probably
emitting at a wavelength in the 635 to 670 nm range. They are found in all
modern red laser pointers, newer barcode scanners, laser light positioning
devices, and now in DVD (Digital Versatile/Video Disc) players and DVDROM
drives.

You can easily destroy the typical laser diode through instantaneous
overcurrent, static discharge, probing them with a VOM, or just looking
at them the wrong way. :-)

By far the easiest way to experiment with these devices is to obtain
complete laser diode modules. Versions are available with both the drive
circuitry and (adjustable) collimating optics. They are more expensive
than raw laser diodes but are also virtually foolproof. Inexpensive laser
pointers are one source for similar devices which may be adequate for your
needs but modifying them could be more of a challenge. See the chapter:
Laser and Parts Sources for suppliers of
both raw laser diodes and laser diode modules.

Any time you are working with laser light you need to be careful with
respect to exposure of a beam to your eyes. This is particularly true
if you collimate the beam as this will result in the lens of your eye
bringing it to a sharp focus with possible instantaneous retinal damage.

Typical currents are in the 30-100 mA range at 1.7 to 2.5 V. However, the power
curve is extremely non-linear. There is a lasing threshold below which there
will be no coherent output (though there may be LED type emission). For a
diode rated at a typical current of 85 mA, the threshold current may be 75 mA.
That 10 mA range is all you have to play with. Go to 86 mA (in this example)
and your laser diode may be history in much less than the blink of an eye.

This is one reason why most applications of laser diodes include optical
sensing to regulate beam power. The third lead on the laser diode package is
connected to an internal optical sensing photodiode used to regulate power
output when used in a feedback circuit which controls your current. This is
very important to achieve any sort of stable long term operation.

You can easily destroy a laser diode by exceeding the safe current even for
an instant. It is critical to the life of the laser diode that under no
circumstances do you exceed the safe current limit even for a microsecond!

In addition, as the temperature of the laser diode changes (heats while
powered), the current requirement to produce a given optical output increases
as well. Without optical feedback if you set the current to be correct once
the temperature of the laser diode stabilizes, it will likely blow out
instantly the next you turn it on from a cold start!

Laser diodes are also extremely static sensitive, so take appropriate
precautions when handling and soldering. Also, do not try to test them with
an analog VOM which could on the low ohms scale supply too much current.

It is possible to drive laser diodes with a DC supply and resistor, but unless
you know the precise value needed or have a laser power meter at your disposal,
you can easily exceed the ratings before you realize it.

You might hear someone bragging "I have driven thousands of laser diodes by
just connecting them to a battery and resistor and never have blown any".
Sure, right. While it is quite possible that the susceptibility to instant
damage due to overcurrent varies with the type of laser diode, unless you know
the precise behavior, you must err on the side of caution. Some designers
have gone to extremes, however. See the section: Laser Diode Power Supply 2 (RE-LD2)
for a design with 5 levels of protection!

For an actual application, you should use the optical feedback to regulate
beam power. You should also use a heat sink if you do not already have the
laser diode mounted on one. See the chapter: Laser Diode Power Supplies.

The raw beam from a laser diode is generally wedge shaped - 10 x 30 degrees is
a typical divergence. You will need a short focal length convex lens to produce
anything approaching a collimated beam. The optics from a dead CD player (even
though CD players and CDROM drives use infra-red laser diodes, the optics
can likely still be used with visible laser diodes), a low to medium power
microscope objective, or even an old disc camera can provide a lens that may
be entirely suitable for your needs.

The major difference between these and the visible laser diodes discussed
in the section: Low Power Visible Laser Diodes is that the output is
near-IR - usually at 780 nm (wavelengths from 400 to 700 nm are generally
considered the visible portion of the electromagnetic spectrum). Therefore,
the emission is not readily visible and you must use an IR detector device to
even confirm that the laser is operating properly. This also means that
safety is even more of a consideration with these devices since what you
cannot see CAN hurt you (or at least your vision).

Thus, these devices make truly lousy laser pointers or laser light shows as
the emission is just barely visible in subdued light. If you hoped for a Star
Wars type laser beam, better go hunting for a 25 W argon laser. :-)

However, for data or voice communications, various kinds of scanning or
sensing, and electro-optic applications where visibility is not needed or not
desirable, such low cost sources of coherent light are ideal.

Similar types are found in CDROM drives and newer LD (LaserDisc) players.
CD-R recorders, Minidisc equipment, magneto-optical, and other writable
optical drives including WORM drives, use devices that are similar in
appearance and drive requirements but may be capable of somewhat higher
maximum power output - as much as 30 mW or more.

Modern laser printers use laser diodes producing anywhere from 5 mW to 50 mW
and beyond depending on their resolution and speed (pages per minute). High
resolution laser imagers, typesetters, and plotters, may use laser diodes
producing 150 mW or more. (However, equipment built before 1985 or so may use
helium-neon or even argon lasers rather than diode lasers.)

The laser diode in a laser printer is located inside the scanner unit which is
probably a black plastic case about 6 or 8 inches on a side and a couple of
inches thick with a motor protruding from the bottom. The laser diode is
mounted (along with its driver board, collimating optics, and even possibly a
Peltier solid state cooler on some) either near one corner or inside. There
should be a laser safety sticker on it as well - but these fall off sometimes!

It is essential that additional precautions are taken if you have a higher
power laser diode from equipment of this sort (or don't really know where
yours spent its earlier life).

There are now laser diodes (or possibly laser diode arrays) with optical
output measured in 10s, even 100s of watts though these will not be what you
would call tiny and will probably require buss bars for electrical power and
plumbing for cooling!

This Laser Printer Diode Laser Module is from an
older unidentified laser printer, laser scanner/duplicator, or similar device.
It shows an example of a typical assembly consisting of an IR laser diode,
collimating optics, and electronics driver board.

Collimation optics: Mini-optic rail with a C/CX or 3 element collimator,
and an f theta lens. These optics (except the 3 element type) work with
laser diodes up to 150 mW and 635 nm wavelength. The 3 element type works
with 670 to 1300 nm laser diodes as well.

Note that this is only the front-end. It does not include the beam scanner
(motor driven multifaceted mirror), field correction and directing optics, or
beam position sensor - which would be present in a complete laser printer.
The output of this module is a collimated IR laser beam. The actual focal
point will be at the image plane (photosensitive drum surface) after passing
through the other optics.

Unless otherwise noted, the following discussion assumes the type of laser
diode found in a CD player or CDROM drive. These are the most common devices
you are likely to encounter. In fact, I bet you have at least one broken
CD player or CDROM drive sitting in your junk box - or maybe you just retired
your 16X CDROM drive because it was soooo slow and obsolete. :-)

CD player laser diodes are infrared (IR) emitters, usually 780 nm, with a
maximum power output of around 5 mW. Their emission will appear very slightly
visible and deep red. This is the eye's response to the near-IR radiation but
appearing about 10,000 times weaker than the actual beam would be it it's
wavelength were centered in the visible part of the spectrum. Despite what
the EM spectrum charts show, the eye's response does not drop off to zero at
exactly 700 nm - there is decreasing sensitivity which may extend out beyond
820 nm depending on the individual (though some people can't even see the 780
nm). Just realize that the main beam is IR and almost totally invisible.
Take care. A collimated 5 mW beam is potentially hazardous to your eyes.
Don't be misled into thinking the laser is weak due to the dim appearance
of the beam. It is not supposed to be visible at all!

If you don't want to take even the minimal risk of looking into the lens at
all, project the beam onto a piece of paper held close to the lens. In a
dark room, it should be possible to detect a red spot on the paper when the
laser is powered. For any laser more powerful than this or where the beam
may be even approximately collimated, viewing the spot on a diffuse surface
is the only safe method for checking the beam.

Typical CD laser optics put out about 0.3 to 1 mW at the objective lens though
the diodes themselves may be capable of up to 4 or 5 mW depending on type.
If you saved the optical components, these may be useful in generating a
collimated or focused beam. The aspheric objective lens will be optimized
for producing a diffraction limited spot about 1 to 3 mm from its front
surface when the optical system is used intact.

The optics may include a collimating lens, diffraction grating (to produce the
three beams in a three beam pickup), beamsplitter prism or mirror, turning
mirror (for horizontally mounted optics), and focusing (objective) lens.
Older pickups tend to have larger and more complex sets of optics. Despite
the fact that they are mass produced at low cost, these are all very high
quality optical assemblies.

However, depending on design, some of the parts may be missing or combined
into one component. For example, many Sony pickups do not appear to use a
collimating lens. For pickups with a collimating lens, if the objective
lens is removed, you should get a more or less parallel main beam and two
weaker side beams. Many newer designs have a combined laser diode/photodiode
array rather than individual components. Mix and match parts for your needs
(if you can get it apart non-destructively). Where there is no collimating
lens, the objective lens may be used for this purpose if positioned closer
to the laser diode.

Typical drive currents are in the 30 to 100 mA range at 1.7 to 2.5 V. However,
the power curve is quite non-linear (though perhaps not as extreme as the
typical visible laser diode). There is a lasing threshold below which there
will be no coherent output (just IR LED emission). For a diode rated at a
nominal current of 50 mA (typical for Sony pickups, for example), the threshold
current may be 30 mA. This is one reason why most applications of laser diodes
include optical sensing (there is a built in photodiode in the same case as
the laser emitter) to regulate beam power. You can easily destroy a laser
diode by exceeding the safe current even for an instant. It is critical to the
life of the laser diode that under no circumstances do you exceed the safe
current limit even for a microsecond!

Laser diodes are also supposed to be extremely static sensitive, so use
appropriate precautions. Also, do not try to test them with an analog VOM
which in particular could on the low ohms scale supply too much current.

It is possible to drive laser diodes with a DC supply and resistor, but unless
you know the precise value needed, you can easily exceed the ratings.

For an actual application, you should use the optical feedback to regulate
beam power. You should also use a heat sink if you do not already have the
laser diode mounted on one. CD laser diodes are designed for continuous
operation. See the chapter: Laser Diode
Power Supplies.

Some manufacturers of CD and DVD optical pickups have gone to a combined laser
diode/photodiode (LD/PD) array package which looks like a large LD but with 8
to 10 pins. Aside from the objective lens assembly, the only other part may
be the turning mirror, and even this is really not needed. Such a pickup can
be very light in weight (which is good for fast-access drives) and extremely
compact.

Eliminating the components needed to separate the outgoing and return beams
should result in substantial improvement in optical performance. The only
disadvantage would be that the beams are no longer perfectly perpendicular
to the disc 'pits' surface and this may result in a very slight, probably
negligible reduction in detected signal quality - more than made up for by
the increased signal level.

Some of these use what are known as "hologram lasers" (a designation perhaps
coined by Sharp Corporation). With
these, the functions previously performed by multiple optical components.
can be done by a "Holographic Optical Element" or HOE. The HOE can simply
be a diffraction grating replacement or can be designed to perform some more
complex beam forming. A variety of hologram lasers (as well as conventional
laser diodes and photodiode arrays) used to be listed at the Sharp Web
site. I do not know if they are still manufactured.
The typical Sharp hologram laser (versions for CD, DVD,
and other types of optical storage devices) eliminate the normal diffraction
grating in the three-beam pickup as well as the polarizing beamsplitter and
associated components making for a very simple, compact, low cost unit.

The photodiode's forward voltage drop will be in the approximately 0.7 V range
compared to 1.7 to 2.5 V for a red visible or near-IR laser diode, up to
6 V for a Nichi blue/violet LD. So, for the test below if you
get a forward voltage drop of under a volt, you are on the photodiode leads.
If your voltage goes above 3 V, you have the polarity backwards.

CAUTION: Some laser diodes have very low reverse voltage ratings (e.g., 2 V)
and will be destroyed by modest reverse voltage at a few microamps of current.
Check your spec sheet. However, the laser diodes found in CD players seem to
be happy with 4 or 5 volts applied in reverse. Of course, a shorted or open
reading could indicate a defective laser diode or photodiode.

If the laser diode is still connected to its circuitry (probably a printed flex
cable), it is likely that the laser diode will have a small capacitor directly
across its terminals and the optical sensing photodiode will be connected to a
resistor or potentiometer. In particular, this is true of Sony pickups and
may help to identify the correct hookup.

And finally, determining pinout without applying power to the laser diode
package is possible by taking advantage of the sensitivity of the
laser diode (LD) and photodiode (PD) to external light. However, once the
tests below have been performed, it's probably a good idea to confirm with an
ohmmeter or some other technique.

A light source with a wavelength shorter than that of the laser diode
must be used, so this could be problematic for violet laser diodes,
but for red or IR LDs, a green laser pointer or flashlight works well.

But it must be taken with a grain of GaAsP :) as I've seen some strange
behavior on some laser diodes.
In particular, in testing a high power laser diode - 20 W, 19 emitters,
shining a green laser pointer or flashlight on the output facet produced the
expected result - up to a few hundred mV with the positive on the anode
of the diode (the + input). However, shining the same light source in
from the *side* sometimes produced a *negative* voltage of 100 mV or more!
What's the explanation for that?

It did work as expected with a 9 mm can package.
Of course, this does assume that the pins are known to be for
the laser diode and not a monitor photodiode or TEC!! :)

(From: Nikos Aravantinos (aravantinos@ath.forthnet.gr).)

After having played with several CD and CD/RW diodes, I believe that it is
possible to determine the pinout to a high degree of confidence without
applying any significant power to the laser diode.

All that is needed is a voltmeter (rather a millivoltmeter) and an operating
incandescent lamp (tungsten filament like a pocket flashlight). If you direct a
light beam to the device under test and measure the voltage between common and
each of the other two pins you will find two of the four following
possibilities:

About +500 mV. This is a PD anode.

About -500 mV. This is a PD cathode.

About +5 mV. This is a LD anode.

About -5 mV. This is a LD cathode.

The large difference is due to the fact that the photodiode is a much more
efficient converter of light to electricity although both the PD and LD work
as photo cells. The above figures depend on the intensity of the light but
there will be no mistake: The PD voltage will always be much larger that the
LD voltage.

If your power supply has a current limiter, set it at 20 or 25 mA to start.
You can always increase it later. If a suitable bench power supply isn't
available, one which can be built for a few dollars and has the needed bells
and whistles is described in the section:
Sam's Laser Diode Test Supply 1.

R2 limits the maximum current. If you know the specs for your diode, this
is a good idea (and to protect your power supply as well). You can always
reduce its value if your laser diode requires more than about 85 mA (with
R2 = 100 ohms).

The two capacitors provide some filtering to reduce the risk of a transient
blowing the laser diode. C2 should be mounted close to the laser diode.
The part about 'no overshoot' is very important. If the supply isn't well
behaved, it will fry laser diodes. See the section:
Testing of Laser Diodes Using a Lab Power
Supply for additional comments.

Before attempting to obtain lasing action with either of these circuits,
monitor the voltage across what you think is the laser diode as you slowly
increase the power supply or potentiometer.

If you guessed correctly (or have the pinout diagram from the spec sheet
or determined from its former life), the voltage will increase until around
1.5 to 2 V and then climb more slowly. Don't push your luck unless you are
also monitoring the laser diode current and optical output.

If you are across the laser diode or photodiode in the reverse biased
direction, the voltage will continue to climb above 2 V without slowing.
Don't push your luck here - the breakdown voltage of the laser diode may
be only a little more than this and - you guessed it - exceeding this is
not healthy for the laser diode either.

If you are on the photodiode in the forward direction, the voltage will get
stuck around .7 V.

Once you have identified the correct connections, very carefully monitor the
current through the laser diode as you slowly increase the current and check
for a laser beam:

For IR laser diodes, you *must* use an IR detector circuit, card, video
camera or camcorder (with the requisite 3 hands) to monitor for an actual
IR laser beam. See the section: Methods of
Sensing IR for a variety of options.

For visible laser diodes, you can use your eyeballs or any more
sophisticated detector as desired. Look from an oblique angle or better yet,
place a white card a couple of inches in front of the laser diode. Even a
1 mW laser diode is an intense source of light - there will be no doubt when
lasing begins.

Some typical operating currents for laser diodes of various wavelengths are
listed below. THESE ARE JUST EXAMPLES. Your laser diode may have a lower
operating current than the ones listed here! The lasing threshold may be as
little as 5 or 10 mA below the operating current and the operating current may
be 5 mA or less below the maximum current.

However, some laser diodes may have an operating current as low as 20 mA and
VCSELs tend to be much lower (but you probably don't have any of those
to play with yet!).

Of course, if you inherited a bag of identical laser diodes and can afford to
blow one: (1) I could use a few before you do this :-) and (2) you probably
could fairly accurately characterize them by testing one to destruction.

For a current below the lasing threshold for your laser diode, there will be
some emission due to simple LED action. As you slowly increase the current,
at some point (if the laser diode is good) as you exceed the threshold current,
the character of the emission will change dramatically and a very slight
increase in laser diode current will result in a significant increase in
intensity. Congratulations! The laser diode is lasing.

CAUTION: unless you have a laser power meter, don't push your luck. The
maximum safe current may be as little as 5% above the lasing threshold. Go
over by 6% and your diode may be history. The exponential power curve seems
to be steeper with visible laser diodes but there is no way to be sure without
specifications. It is all too easy to convert laser diodes into extremely
useless DELDs (Dark Emitting Laser Diodes) or very expensive LEDs.

I have used this approach with laser diodes from dead CD players without
difficulty. In the case of many of these, the operating current is printed
on a sticker on the optical block, often as a 3 digit number representing
the current in 10ths of mAs. Typical values are 35 to 60 mA (350 to 600).
Sony pickups typically average around 50 mA. Without this information, the
best you can do is to estimate when it is lasing at the proper intensity by
comparing the brightness of the 'red dot' one sees by looking into the lens
from a safe distance at an oblique angle. However, this is not very reliable
as the optical power at the objective lens depends on the particular CD player.

Even if you have complete test data for you diode, it's still a good idea to
start low and monitor output power. The diode was originally tested under very
precise conditions which probably aren't quite the same as you have (e.g.,
temperature) so laser diode or monitor photodiode current could be different
by enough to cause problems.

Attempting any of the following may result in total destruction of your laser
diode, but if you are willing to risk its health, there may be a way of
determining something about where damage will occur and possibly have it
survive more or less intact.

If the failure mechanism for your particular laser diode is NOT Catastrophic
Optic Damage (COD) to the facets but something else like thermal damage,
then it may be possible to identify the onset without
serious harm by looking for a fall off in slope efficiency. For some types
of laser diodes, the rate of increase of output power with respect to
drive current will decrease well before there is a noticeable - or any -
permanent loss of performance or that magic transformation to a
Dark Emitting Laser Diode (DELD) or expensive LED. :)
But there is no way to know if COD is the limiting failure mode for
any particular laser diode without testing it, possibly to destruction.
If the limiting damage mechanism is COD, there may be no indications of
distress before the creation of a DELD.

This testing is best done with the diode on a good heatsink or TEC. Increase
current in small increments while monitoring output power. After the onset
of lasing, the output power should increase quite linearly with current.
But near the limit, this slope may decrease. Stop there!
A well behaved curve tracer (no overshoot or glitches, etc.) can also be
used since then the onset of non-linearity will be very obvious on the
graphical display as the peak current is increased. But note that a high
speed curve tracer may actually side step the thermal issues until COD occurs
and it is too late, because the short time it spends at the highest current
doesn't allow for a significant temperature increase in the laser diode.

Even though the output power is still increasing after the slope changes,
don't go there beyond there! You'll be treading on dangerous ground. Of
course, it's possible that some latent damage has already occurred by the
time any noticeable non-linearity is seen so no guarantees if trying such
a stunt.

All reasonably civil comments are welcome. ;-)

(From: Lostgallifreyan.)

I've learned to detect the onset of critical overdrive by eye. :) I'm not
sure it always works, especially on the higher power single mode diodes,
but it works with the old gain guided Philips OF4944's and the newer
Hitachi/Opnext 35 and 50 mW 658 nm index guided MQW diodes.

When you look at the projected spot on a dark surface, the appearance of
the light goes from strongly specular to a less specular output as you
approach destructive drive levels. I haven't got the kind of tools needed
to quantify what is happening but I think the line broadens, or more likely
becomes noisily erratic the way audio filters with feedback become sine
wave generators, moving up from noise to clear sine like a laser does at
threshold, and then producing a keen abrasive sounding edge if you apply
too much gain. I'm thinking this is maybe a good analogy, and that the
effect of too much input is visible as increased noise.

Note that the critical limit is VERY close above that visible noise
threshold. I've often saved a diode for long term running at WAY over
recommended max current, by detecting this by eye, then backing off until
the light is strongly specular again. I've found one weakness to this
though, it is best to use on lower powered diodes, max 50 mW, and the
better ones at that. If you use the cheapest for a given power, you'll find
inconsistencies, especially regarding risk of instant DELD after strong
retro-reflection. This is no big deal though, the high power single mode
diodes will always die from that even under correct operating conditions.
Cheap diodes might be broader in linewidth anyway, so it might be harder to
see the critical increase in noise.

This applies mostly to high power laser diodes such as those used for solid
state laser pumping. However, measuring the current of a 5 mW laser pointer
diode can sometimes come in handy. :)

Usually, there will be a current test point in the power supply with a
specified calibration in terms of volts/amp of diode current. Of course the
circuit could be defective resulting in incorrect readings.

So, ultimately, it will come down to putting a current meter in series with
the LD unless you have a clamp-on DC ammeter (which isn't common). As
long as it is a decent instrument with adequately sized short leads (e.g., no
significant voltage drop) AND you make all connections securely with power
off and using proper anti-ESD practices, there should be minimal risk to the
diode. Just remember that most high power laser diodes have their positive
terminal bonded to the heatsink and this is generally grounded so the meter
must be isolated.

If there is a series resistor already present, measuring the voltage drop
across it and computing current as V/R is quite acceptable. Again, make
all connections in a secure manner with power off. Double check that your
meter is set to a voltage range NOT CURRENT as that would result in
a low shunt resistance across the existing resistor and if that is used for
current sensing, would increase the current through the laser diode -
possibly to destructive levels.

Adding a series resistor so measurements can be made in this manner is also
possible though more risky. It must be a low enough value so as not to affect
the behavior of the driver circuit. Some drivers may be affected by the
actual diode voltage even if it only varies by a few dozen mV. A true
constant current driver won't care.

Sorting by noticeable differences is almost useless - later model 40 milliwatt
diodes come in the 5 mm package now. You can't tell much by looking at the
packages!

My experience has been that lasing threshold current can vary by a factor of 2
(with temperature and this is verified by the Sharp catalog). Threshold
current is NOT any sort of reliable indicator - that's why the drive
electronics senses actual optical power output!

That's NOT to say that knowing the threshold isn't useful.

Here's my take on it:

I think that once the threshold has been reached, you can push the diode to
about 10 percent past that current safely. For bigger diodes, you probably
have 20 percent + of cushion.

Let's say I have a diode that snaps to laser mode at 50 mA. I'd drive it to
55 mA and measure the output quickly. I would set my APC to maintain that
power level output and go on to the next diode.

For larger diodes, it's common to not even use a feedback photodiode for
power sensing. Thats because these diodes have MUCH wider margins between
the threshold and the smoke valve release ratings. Let's say I find a 2
lead LD that starts lasing at 400 mA. This diode can probably be pushed an
additional 20 to 25 percent and driven with a constant current source.

With no name/unspecified diodes, in my opinion I'd stick with making them lase
and holding them at that power output rather than squeezing every last
milliwatt from them.

I might loose a few in testing, but I surely would not loose many.

Use a large area PD mounted right on the face of the LD under test. You can
use a bias supply and a series resistor. Put your voltmeter across the
resistor. As you slowly ramp up the LD current, you will see all hell break
loose when observing the power output meter. Above threshold, the LD is
fairly efficient and fairly linear (power out versus current above threshold).

As a ball park figure, you can assume that the threshold current is about 10
to 15 percent of maximum power out for the diode although it varies a lot
for bigger and for IR diodes. So, trying to operate a LD to maintain 5
percent of it rated output is damn close to impossible because of the
nature of the beast.

Again, all figures and numbers quoted widely variable. Don't take them
too seriously.

PS: Make sure your LD testing supply is smooth (ramp up) and test it with
an LED first!

The following is just a microscopic sample of data for some unidentified
visible (red) laser diodes from my (anti-static) junk box.

Having analyzed the circuit in the section:
Laser Diode Power Supply 4 (RE-LD4), I
then proceeded to try out a variety of typical visible laser diodes. For
all the undamaged laser diodes that I tested, leaving SBT open resulted in
safe feedback regulated operation at Vcc1 = Vcc2 = 7 V. But, depending on
the particular sample's photodiode sensitivity, optical output power varied
widely.

While testing, I used a regulated power supply with adjustable current limit.
The voltage was set at 7 V and the current limit knob was used to ramp up the
input to the driver while monitoring laser diode current and/or feedback
voltage from the photodiode. This approach may have prevented damage to a
laser diode on more than one occasion.

The numbers in () do not mean anything - they were found marked on each sample
and are only used to identify them uniquely.

Laser output power was estimated to seven significant digits based on the
perceived brightness using my Mark-I eyeballs (with AutoCal(tm) option). :-)

The resistance of SBT (R7) is listed. However, the actual photodiode load
is R7||R6 (33.2K) and thus the photodiode current is (Vcc1/2) = 3.5/(R7||R6)
when optical feedback is successful in maintaining regulation. Since the
photodiode current should be proportional to optical power, you will probably
find that my high mileage eyeballs suffer from some slight non-linearity as
well. ;-)

I do not have specifications for any of these laser diodes. However, they
are typical of the 660 to 670 nm types capable of 3 to 5 mW maximum output
power found in readily available diode laser modules and laser pointers.

Samples 1 through 6 were all in a large (9 mm diameter) package while samples
7 through 9 were in a small (6 mm diameter) package. As you will note, for
these types of laser diodes, power output does not really correlate with
package size. Each was mounted along with a collimating lens (adjustable in
some cases) in an aluminum block or cylinder (variety of styles) which also
acts as a heat sink.

I suspect that samples 2 and 3 were of similar construction but that this
differed from that of samples 1 and 4. Note how sensitive sample 1 is to
slight increases in current - dramatic evidence of the risks involved in
running these without optical feedback. Samples 7 through 9 also appeared to
be similar but I only had one fully operational unit of this type to test so
no detailed comparison could be made.

I do not know whether the higher current for sample 2 is due to prior damage
or just a normal variation in laser diode power sensitivity.

Samples 4, 8, and 9 (*) had been damaged to varying degrees previously due to
running with excessive current. These disasters occurred prior to analyzing
the behavior of this laser driver circuit. Sample 9 was absolutely positively
beyond a shadow of a doubt totally dead laser-wise behaving like a poor excuse
for a visible LED in a cool-looking fancy package. :-)

In the case of samples 5 and 6, I continued to decrease SBT until a distinct
jump in laser diode current was required to maintain the voltage across SBT
(and thus beam power). For example, with sample 5, the jump from 74 mA to 89
mA may have indicated that losses were building and damage or total failure
would have resulted if pushed any further. However, at that point, no changes
in laser diode behavior had occurred and all lower power levels ran at the
same drive current as before. Note: I do not know if this is a valid approach
for checking the limits of a laser diode but it may work for some types.

All of the other (undamaged) laser diodes tested could probably have been
pushed to higher output power but without knowing their precise specifications
and only using my Mark-I eyeballs for a laser power meter, I chickened out.
However, there was definitely headroom above the power levels listed above.

I was asked to test a bunch of Nichia NLHV500B/C 5 mW violet laser diodes,
with wavelengths between 398 and 410 nm.
Fortunately, I was able to use a high quality laser diode controller - the
ILX Lightwave LDC-3900 with a 500 mA driver module. This has enough voltage
compliance range for the 4 to 5 V across the diode at its operating current.
In most respects, aside from the peculiar color, these diodes behave more or
less just like any others. However, there are a few items to note:

The appearance of the output (to me at least) was more of a white-ish blue
than the deep violet I expected. Certainly a lot of this was fluorescence
of the white paper used as a screen but almost anywhere the beam fell, it
had this appearance. Apparently, nearly everything fluoresces at these
short wavelengths.

About 10 percent of the output power when measured with my LaserCheck
laser power meter was actually in LED emission. So, a reading of 5.5 mW
on the LaserCheck was really only 5 mW of laser output and 0.5 mW of LED
output. However, the monitor photodiode was accurate, agreeing with the
data printed on the box.

The beam pattern looks pretty much like that of any other typical laser
diode - spread out in the fast-axis, relatively narrow in the slow-axis.
But it appears to have numerous striations which I don't believe are
due to damage or dirt since they are very fine - almost like speckle but only
in the fast-axis direction - and are similar on all the diodes that
actually lased. There is also a lot of off axis scatter that I've heard
is normal for Nichia diodes.

A heatsink is essential for stable operation. Since the heat dissipation
is relatively low (200 mW typical), a TEC isn't needed for testing where
stability isn't important but without even a heatsink, power would drop
off very quickly and even lasing became erratic.

Someone who didn't have a clue about testing laser diodes had gotten to
these before me but apparently wasn't able to destroy them all. That
in itself was amazing. :)

Out of 9 samples:

Three worked with threshold, operating, and monitor current close to the
values printed on the box.

One worked with very reasonable currents but obviously wasn't the
device originally in the box since the monitor current wasn't correct.

One had a somewhat high threshold and operating current but still
achieved a stable 5 mW of output. It may still be in spec but not
actually be the device from the box that it was in. (The diode package
is only marked with a laser enscribed nearly microscopic 2-D barcode
which can't be deciphered without a scanner. And even on units where
I'm sure the diode is in the proper box, there appears to be no obvious
correlation between the barcode on the diode and the barcode on the box
label!)

Two were almost totally dead with no LED emission until near 100 mA.
Even this was erratic. The electrical characteristics of these showed
a high leakage current so the operarting voltage of 4 or 5 V or more
needed for efficient LED emission or violet lasing wasn't reached.

Two worked as LEDs but didn't lase even at 100 mA. Taking one of these
to 150 mA resulted in it joining the ranks of the dead ones, above.

While I haven't actually looked at the longitudinal mode structure of
coherence length, here is some info:

(From: "Lynn Strickland" (stricks760@earthlink.net).)

We're coupling Nichia diodes to single mode fibers. Our key program
engineer says that lasing on multiple modes and mode hopping is a big
problem with Nichia diodes. They are not single mode and tend to jump as
much as 1 nm away in wavelength without warning. He doesn't think Nichia
diodes will ever work in an application requiring single frequency light
unless someone makes a breakthrough.

These typically come in a 14 pin package similar to a DIP IC with leads
sticking out the sides. They are supposed to be mounted in something called
not surprisingly, a "laser diode mount" for testing, but of course you
don't have one of those.

An example of this type of unit is the CQF938 from
JDS Uniphase. This exact model
number is no longer listed on the JDSU Web site but may be found
at Uniphase CQF938
High Power 1,550 nm CW DFB Lasers with PM Fiber. It includes a DFB laser
diode with photodiode power monitor coupled into a polarization-maintaining
fiber, bias T LD drive network, and Thermo-Electric Cooler (TEC)
and temperature sensor thermistor.

If a laser diode mount is not being used, the package will have to be
clamped to a good heatsink. Based on the pinouts found on the datasheet,
the TEC controller will drive pins 6 and 7 for the TEC+ and TEC-,
respectively. The sensor is pins 1 and 2 with the controller set for
a 10K ohm thermistor.

The DC drive to the laser diode is on pins 11,13 and 3 for its anode
and cathode, respectively. The modulation is AC coupled in via pin 12.
If optical feedback for output power regulation is to be used,
the monitor photodiode is on pins 4 and 5.

And, despite it being in a fancy, and very expensive package,
extreme care must be taken in handling and drive as the
laser diode is still sensitive to EVERYTHING!!!

The laser diodes in CD, DVD, HD-DVD, and Blu-ray burners typically do not
have an internal monitor photodiode. So, for most hobbyists, this means
the only practical way of powering them is with a constant current supply.
At least, that's what can be done for testing. Once installed in a
permanent setup, an external monitor photodiode can be added to implement
constant output power operation, but that's for the advanced course. :)

These laser diodes are operated at two different power levels - low power
(less than 5 mW) for reading and high power (30 mW and up) for burning.
I assume that there is some external monitoring of the power to regulate
this in the DVD burner, but it's not inside the laser diode.

If the specs are known, then using a heavily filtered well behaved
(no spikes, overvoltage, or reverse polarity when power cycling or due
to line transients!) adjustable voltage power supply and series current
limiting resistor is probably easiest.

The laser diode should be mounted in a heatsink. Leaving it in the original
mounting of the burner is acceptable as is clamping the can between a pair
of aluminum plates, one with a hole drilled through it.

For the IR and red LDs from CD and DVD burners, respectively, the polarity
can be determined in the usual way if a spec sheet isn't available - by
increasing the voltage *very* slowly (with the current limiting resistor)
up to 1.5 to 2 V but NO MORE. The LD will
start conducting by then if the polarity is correct. For HD-DVD and Blu-ray
LDs, it's really best to check specs since the maximum reverse voltage may
be lower than the minimum forward voltage where conduction begins.

Once the polarity is known, slowly increase the voltage while monitoring
current and output power As usual, the LD will behave as an LED up to its
lasing threshold with a somewhat diffuse glow, and then the rate of change
of output power will dramatically increase above threshold, with a narrowing
of the beam pattern.

Some of these LDs are good for 100 to 200 mW or more of single spatial
mode output - especially high-X DVD burner LDs. But without the specs,
there is no way to know when they will start turning into DELDs (Dark
Emitting Laser Diodes).

Once the operating point is known, a power supply can be built either using
the same approach of a constant voltage through a series current limiting
resistor, or with an IC regulator like an LM317 in constant current mode.
However, I prefer the former as it's more difficult for misbehavior to
zap the laser diode since the maximum current will be limited by passive
components rather than the IC, doing who-knows-what when power cycled.

Checking with other people who have already played with these LDs is a good
idea. One place to ask is on the USENET newsgroup alt.lasers. Another
would be the various holography forums. They may already have
discovered the limits of your specific model of burner LD (possibly
the hard way).

The following applies to the sorts of laser diodes that are used to pump
solid state lasers and provide large amounts of heat or light in a small
area for medical or materials processing applications. They are not what's
found in a laser pointer!

WARNING: With multiple WATTs of output power, particularly for high
power IR laser diodes, both eye safety and even possible heat/fire damage to
materials must be taken seriously. NEVER look directly toward the
output end of the laser diode unless there is no chance of any power being
applied to it (even from residual capacitor charge). Direct the output in
such a way that it isn't possible to for any eyeballs to intercept the beam
or specular reflections. If the beam isn't focused, the heat/fire damage risk
is minimal but something to take into consideration. Near-IR laser diode
output may look weak and whimpy but realize that the actual intensity is
10s of thousands of times stronger than it appears!. Although the output of
a bare laser diode diverges greatly, if fitted with any sort of collimating
optics, the mostly invisible beam remains dangerous for a distance of many
feet. Become complacent around these and your vision is at serious risk.

However, to maintain one's respect for these things, it would be nice to
pop in an equivalent power visible source. Of course, for a truly high power
laser diode array - say a 60 W 808 nm bar - this is basically impossible
though if you think of all the light being emitted from a 1,000 W light bulb
but with a source size of 1 cm or so, it won't be far off. But an
appreciation could be gotten even for a 0.5 W source by substituting a 0.5 W
visible laser diode (if you can afford one!) and realizing how darn bright
and concentrated it really is!

Also, even an output power as low as 10 mW is enough to affect dark materials
when focused with even a simple lens. The beam from a 30 mW laser diode will
easily melt black electrical tape and put tiny holes in paper and wood
surfaces.

These may be in a TO3 or other transistor-like case or a standard or
non-standard laser diode package which may also include a TE cooler (TEC)
and temperature sensor. The laser output may be via a window, collimating
or focusing lens, or fiber. But the assumption is that there are no
electronics inside for the laser diode itself beyond possibly a bypass
capacitor and/or resistor, and reverse protection diode.

As with ALL laser diodes, locating a datasheet with pinout is truly the
best solution. Where there is a manufacturer's part number, a Web search
may be successful. Even if the exact model isn't found, the package may
be sufficiently standard that a close match will be sufficient.

If there are only two terminals or wires, then all that needs to be determined
is which one is the anode and which one is the cathode of the laser diode.
In most cases, the anode (+) will be connected to the metal case. This
can be tested with a DMM on the low ohms range (not an analog VOM which
may produce too much voltage/current and damage the laser diode).

For those with more than 2 connections, there is likely an internal TE
Cooler (TEC) and associated temperature sensor so expect 3 pairs of wires or
terminals. Some may be twisted pairs or coax but that really doesn't help
much. Even which are paired may not be obvious so checking of all possible
combinations may be necessary.

Measure between pairs, again using a DMM only:

The temperature sensor is usually an NTC thermistor with a resistance of
between 5K and 15K ohms at 25 °C. Most common is around 10K ohms.

The TEC will measure a few ohms and will read differently depending on
which probe is on which terminal since it generates a few mV if its two sides
are at different temperatures as they would be from handling. With some
DMMs, readings may not even make sense due to the generated voltage. Or,
measure its voltage on the DMM's mV range. Nothing else in the package
will produce a voltage output of any consequence.

The polarity of the TEC can be determined by monitoring the sensor with
a multimeter while passing a small current through the TEC (100 mA should
be more than enough to detect a change). With the polarity correct for
cooling, the resistance of the NTC thermistor will increase. When heating,
it will decrease. Try both polarities checking for an opposite change in
the sensor resistance to confirm that the TEC is what's actually being
driven.

The laser diode may or may not have its anode connected to the case since
the TEC will be physically between the diode and the case. In some cases,
the LD can be identified using the diode check function of your DMM but ONLY
if its open circuit output voltage is less than 2 V!! and only if any reverse
protection diode, if present, is silicon, not another laser diode chip
(probably without mirror coatings) as are used by some manufacturers. It
should read either 0.7 V or open in the reverse direction (0.7 being if
there is a silicon reverse protection diode, open if there is none). It
should read about 1 to 1.4 V in the forward direction if there is no parallel
resistor. If there is, it may take a few dozen mA before the voltage
flattens out in the forward direction. However, your mileage may vary
and knowing the pinout is really best. :)

For powering up, see the sections on testing of high power laser diodes.
Here are very rough guidelines for typical 800 nm to 900 nm non-fiber-coupled
laser diodes:

For fiber-coupled types, there is a loss coupling into the fiber which may
be as high as 50 percent. Even for diodes with microlenses, GRIN lenses,
or normal glass lenses, there is some loss from the the wings of the highly
divergent raw diode beam not making it through the optics.

In many ways testing of 0.5 W and higher laser diodes is easier than low
power types. The main reason is that while overcurrent spikes can still
destroy or damage the laser diode instantly, the difference between the
threshold current and max current is much greater for high power laser
diodes. However, they are definitely susceptible to instant death from
reverse voltage above a few volts and all other handling, ESD, and driving
precautions still apply. A bit of static may not result in instant destruction
but can easily cause a microscopic defect which will only grow with time.
Keep the laser diodes in antistatic (black foam) material when not in use
with their connections shorted together. Make it a habit to touch an earth
ground before touching the laser diode.

The same general testing approach can be followed as with low power devices.
If no high quality adjustable laser diode driver is available, I would suggest
a very simple rectified transformer with very large filter capacitor bank to
minimize ripple. Control this from a Variac. Use a current limiting power
resistor of several ohms between the caps and the diode. Depending on the
size of the laser diode, anywhere between 1 and 10 A may be required. Put
a modest load across its output to discharge the filter caps quickly after
power off. For up to 2 A, I've used a 16 VAC, 5 A power transformer, bridge
rectifier, about 20,000 uF filter capacitor, an 8 ohm, 50 W power resistor,
and a 100 ohm, 5 W load. The reason I suggest using such a simple power
supply is that it is inherently free of overshoot on power cycling (which
can't be said in general for active regulators unless specifically addressed
in the design).

Note that these high power laser diodes usually don't have monitor photodiodes
for optical feedback - output is determined via current and temperature
control. For the purposes of testing, if you have a TEC (Thermo-Electric
Cooler), set it for around 20 °C. If you don't have a TEC, mount
the laser diode package on a large heat sink (with forced air cooling if
necessary) to minimize temperature rise. As long as the laser diode package
itself remains cool or just warm to the touch, it will be fine.

CAUTION: Change connections - including any meters - only with power
OFF and the filter caps of the power supply fully discharged.
Even the charge on a 1 uF, 5 V capacitor can damage a 35 WATT, $10,000
laser diode if it is not current limited to a safe current for the diode!
Make sure the output of the laser diode is pointed safely away from you
but don't put anything right up against the output facet or window - at
these power levels, it may get toasted, especially if a dark color and
this will tend to destroy the diode as well.

Determine the pinout. Almost all the 800 to 1,000 nm, 0.5 to 6 W laser
diodes I've played with have been mounted such that the positive (+) power
supply lead is the case or bussbar. The negative (-) is attached to a
separate contact (which itself connects to the top of the laser diode chip via
multiple gold wire bonds or a thin soldered wire). I've only
come across one exception and the diodes were some kind of specials
labeled "R&D".

If you aren't sure, the best thing to do is locate the specs (!!) or trace
the circuitry of the driver/controller if available. Else, it is possible
to determine the pinout experimentally with little risk:

Start with the assumption that the case/bussbar is positive and the top
or flying lead of the laser diode is negative.

Attach the power supply with current limiting resistor (make sure it's
off and discharged!) and connect a voltmeter on its 2.5 to 5 V scale
directly across the laser diode. Make sure the output end of the laser
diode is facing a safe place just in case you get more than you bargained
for! If all you are stuck with is an expensive variable current source,
put a 100 ohm resistor across the laser diode so that current will be be
converted to voltage (1 V/10 mA) - otherwise, there may be no way to limit
the voltage across the diode before it is too late.

Start at 0 V and slowly increase the voltage while watching the
meter. For IR laser diodes, the voltage across the laser diode should start
leveling off at about 1.5 to 1.7 VDC. (Visible ones will be a little
higher.) If it goes much beyond this, you have the laser diode backwards,
there is a broken wire, or if your package has multiple pins, you don't have
the correct ones. DO NOT go above 2 VDC! You risk destroying the
laser diode totally from reverse breakdown (more than a few microamps will
do it! A typical spec is PIV of 3 V, 25 uA max). Check and/or interchange
connections and try again. If the voltage doesn't go above 0.7 to 1.0 V,
your connections are backwards and there is a reverse protection diode built
into the laser diode assembly. If the voltage doesn't increase very much at
all, either the power supply or meter isn't connected properly or the laser
diode is shorted from severe overload or reverse breakdown (but it might
just be a bent bond wire/strip) or poor soldering job of the connections.

On laser diode packages with multiple pins (e.g., TO3), there are many more
combinations to check but each can be tested in a similar manner. If you
have the driver/controller, tracing its circuitry can greatly narrow the
possibilities.

Now that you know the pinout, some means of detecting the laser diode's
output is needed. For visible laser diodes (up to 700 nm), initial
determination of life/death can be made by eye. For near-IR diodes up to
about 850 nm, the eye still has some sensitivity and the emission will
appear deep red but very faint, even for a high power diode (the longer the
wavelength, the fainter it will appear. Just realize, the actual intensity
may be 10s of THOUSANDS of times greater than it appears!). Beyond
some wavelength, there is absolutely no sensation of light. I know 980 nm is
totally invisible to me but the cutoff for individuals varies. DON'T
be tempted to peer close into the output aperture - it could be the last
thing you see from that previously good eye. For a 1 W laser diode, imagine
all the light from a 20 W incandescent lamp being emitted from a source the
size of a grain of sand!

The safest way to monitor output power is with a proper laser power meter.
An alternative is the IR Detector Circuit.
Position its photodiode sensor an inch or so away from the laser diode's
output. The beam shape is highly astigmatic - 5 to 10 degrees horizontally
but perhaps 40 to 60 degrees vertically. Given the output power of these
laser diodes, even with the sensor intercepting only a small part of the
beam, the detector circuit may be overwhelmed (or literally smoked) quite
quickly.

A very simple way of detecting optical output is to place a piece of black
electrical tape close to (but not touching - a millimeter or so) the front
of the LD; at power levels of a few tens of mW, spots will be melted in the
black absorbing material quite quickly. At higher power levels, white paper
will be charred. CAUTION: Don't let either of these touch the facet of the
LD; at the very least it will be coated with burnt stuff (the power density
is highest there); it may also be permanently damaged.

Start at 0 V and slowly increase the current to the laser diode until
threshold is reached and your detector (eyeballs or meter) show a
significant increase in output power. How far to push it? Again, as with
low power laser diodes, the only way to know for sure is from the spec sheet.
But here are some very rough guidelines for 780 nm to 980 nm laser
diodes (at 25 °C):

So, you inherited a bag of unmarked but big identical, but totally unmarked
laser diodes. Can the safe operating current be determined experimentally.

Well, the answer is: maybe if you are willing to sacrifice one.

(From: Bob.)

As a GENERAL rule of thumb and barring infant mortality, ESD, or any other
manufacturing defects in the laser diode, proper heat sinking:

At 100% rated current most high quality, high power diodes (read American
manufacture, around 808 nm or 980 nm) last for 5,000 to 10,000 hours.

At 200% rated current, they last for tens to hundreds of hours.

At 300% rated current, they generally last less than a minute.

So, yes, you can test a diode to failure by slowly increasing the current
until failure occurs and take the current level that destroys the diode
almost instantly and divide by 3. As far as whether this is an
acceptable way to determine the rated current of the diode, the normally
acceptable way is to have the manufacturer spec a current. :) Keep in mind that
these numbers apply to diode bars and C mounted diodes. Can packages are a
little less efficient in coupling heat away from the diode normally, so they
may die a little quicker than normal. In that case you may be running at a bit
lower than rated current if you divide by 3.

While it's generally obvious when a low power laser diode changes from an LED
to a laser, for high power diodes there is significant power due to LED
emission near the lasing threshold. For example, a typical 1 W laser diode
may produce 10 mW or more of incoherent light at around the lasing threshold.
The easiest way to locate the lasing threshold is with an optical spectrum
analyzer which will show when the narrow lasing line appears above the
broad fluorescence spectrum. Without such an instrument, the lasing threshold
current can really only be estimated. While knowing the exact lasing threshold
is probably not all that critical. it might be needed at least be able to
list the value in a table! :)

What I do is to infer the lasing threshold as follows:

Using an adjustable laser diode driver, locate the approximate current
for lasing by observing where the output starts to increase significantly.

Measure the output power (Pi) at two currents (Ii)
approximately 25 percent and 100 percent above this value. Use values for
I that will make calculations convenient. CAUTION: This assumes a current
of 100 percent above threshold is safe for the diode!

Calculate the slope efficiency:
SE=(P2-P1)/(I2-I1).

Calculate the lasing threshold:
It=I1-(P1/SE).

As an example, consider a diode where the current starts increasing quickly
above 550 mA with P1 of 180 mW at I1 of 0.75 A and
P2 of 540 mW at I1 of 1.25 A. Then, SE=0.72 and
It=500 mA.

Here are some data for the types of IR laser diodes used in graphic arts
platesetters and similar equipment. The wavelength is typically between
820 and 880 nm with 0.5 to 1 W or more maximum output. (High power diodes
operating at more useful wavelengths for DPSS laser pumping like 808 nm and
980 nm will have similar characteristics.) Like other
high power laser diodes, these do not contain any monitor photodiode and are
driven by a constant current power supply. These are often fiber-coupled
laser diodes which may feed a collimating lens assembly. A typical unit
is shown in Typical Presstek Fiber-Coupled Laser
Diode. Note that a substantial fraction of the raw output power of
the laser diode (up to 50 percent) is lost in coupling to the fiber
pigtail. Thus, the specifications for the laser diode itself will show
a higher output power.

As I've written many times: There is no way to know the maximum output
power for reasonable life expectancy of these or any laser diodes without
the manufacturer's specifications or testing several to destruction.
As a very rough rule of thumb, it's possibly safe to power a diode at up
to 4 to 5 times the threshold current if properly cooled. So, for one
that starts lasing at 400 mA, 1,600 to 2,000 mA might be OK and it's possible
some will go much higher. No guarantees and your mileage may vary.

Testing was done using an ILX Lightwave LDC-3900 laser diode controller with
wavelength determined using an Agilent or Ando optical spectrum analyzer if
not listed in the part number. Temperature was set at 20 °C.

The first batch are all fiber coupled with an SMA output connector which
attaches to a collimator as shown in the photo, above.

It's likely that Opto Power (now part of Spectra-Physics) is the manufacturer
of the Presstek diodes and that the OPC-A001-FC and some of the AHHs are
the same model. The internal construction of these Presstek
diodes is identical to that of the Opto Power unit shown in
Typical 1 Watt Fiber-Coupled Diode Laser Showing
Interior Construction. All the Presstek 830 nm diodes appear to
have very similar specs.

Although some people may list these Presstek and Opto Power diodes on eBay
as being rated at 2 watts, they are not. I have tested one of each at currents
significantly greater than the value at 1 W. Neither survived to produce 2 W.
AHH03131-2 reached 1.7 W at 2.75 A and
OPC-A001-FC-2 reached 1.75 W at 3 A. Both suddenly dropped to less than 1/4
of their original output power and stayed there. Note that the "A001"
in the OPC part number generally indicates a maximum power around 1 watt.

The next one is also fiber coupled with an ST output connector. It is
rated at 750 mW.

The following was from a platesetter array of 8 diodes feeding via a focusing
lens (no fiber) into an 8-sided mirror at the center which then redirected
the beams out through a feedback controlled objective lens assembly that
looked sort of like a CD player optical pickup on steroids. (I assume the
intent was to scan 8 lines at once since this arrangement would not be able
to combine them in any useful way.) Each of the diodes was in a socketed
TO3 can package with integral TEC and temperature sensor thermistor.

I believe these are actually similar diodes but I didn't use active cooling
on #1 and since the diode is on an internal TEC, thermal resistance is probably
rather high. The current was turned on, the measurement was made, then
the current was turned off. But even this would likely result in a very
substantial temperature rise. Testing of #2 was done with the diode
temperature maintained at 20 °C and this probably accounts for the
higher power readings. Although the diode might survive
at 2 A or beyond, the TEC was incapable of maintaining 20 °C above
about 1,750 mA though the heatsink was cool to the touch. At 2 A, the
temperature was increasing at about 1 °C per second even with 2.5 A
through the TEC.

There are 4 pins on each side of the package. The two laser diode pins have
contacts which automatically short them to the case when there is no
connector attached. The one closer to the edge of the package is LD+,
the other is LD-. Neither is connected to the case directly, being on an
isolated TEC.

A Polaroid diode in a similar package was only rated 200 mW but I couldn't
make any useful measurements on it because it was dead.

The photo shown in
Fiber-Coupled Laser Diode for Platesetter
is of the assembly (one of up to 32) used in an
ECRM "DesertCat 8", a high speed drum scanner for exposing printing
plate masters in the graphic arts industry. It is a fiber-coupled
laser diode mounted on a heatsink with TEC and thermistor temperature sensor.
The diode in the little round can looks like it is from SDL though I've
heard that Kodak may be the manufacturer of the overall assembly. The
output is via an ST fiber connector.

*The tested Iop value of 1122 mA was printed on the diode assembly.
I assume that was for 750 mW since it agreed with my measurements. This
may not be the maximum output power though (likely rated 1 W).

And here are a few fiber-coupled diodes from SDL which are physically similar
to the one in the ECRM assembly, above:

Although some people may list similar diodes on eBay as having a 2 watt
rating, they are not. I have tested two samples at currents significantly
greater than the value at 1 W. FC715 did survive for a few minutes at least
with 2 W of output at about 2.7 A but FC566 died suddenly at about 2.8 A before
reaching 2 W. It is now a shadow of its former self with a maximum output
of about 100 mW. Thus, it may be possible to get more than 1 W from these
diodes but life expectancy could be short, especially if driven above 2 A.

Not all platesetter devices have only a single laser diode inside. I tested
one that actually had 10 diodes side-by-side with separate anode connections
and individual monitor photodiodes. The total length of the 10 diodes
was less than 1 cm. This has a Kodak nameplate model "A" I think. Here
is the pinout of the two sided PCB edge connector:

I'm not really sure of the way the pin numbering starts so this may be
reversed left-to-right. TH is a 10K thermistor for temperature sensing.
The laser diode package was clamped onto a large fan-cooled heatsink.

The laser diode thresholds were about 450 mA producing 250 mW at 750 mA
for a slope efficiency of about 0.84. I do not know what the rated power
is but the sticker on the laser diode package lists "6.5 W max" for all
10 diodes. So, they are at least 650 mW each. Based on the threshold,
they could easily be double this but no guarantees.

The outputs of the laser diodes are fast-axis corrected and reasonably well
collimated, though a rather elaborate set of beam shaping optics is intended
to bolt on to the laser diode package to ultimately create 10 closely spaced
spots from an 8.8X microscope objective for the platesetter engine. The
unit I tested had two such laser diode/optics assemblies.

These are the type of laser diode that don't even reach lasing threshold
until 8 AMPS or in some cases, 16 A or more! Maximum output power may be
10 or 20 or 50 WATTS - or more. They are actually laser diode
bars or arrays consisting of several dozen multimode laser diodes on the
same piece of semiconductor and thus behave like multiple diodes side-by-side
(or in really high power cases, multiple of these sandwiched together).

If you have access to a commercial laser diode controller capable of
20 or 30 or 60 A, great! For the rest of us, there are reasonably
safe (for the laser diode, that is!) alternatives.

What I have used is a high current switchmode power supply intended for
large TTL digital systems. It regulates well at any load and
is capable of 50 A at 5 VDC. I also have one
that will do 150 A if needed. :) Make sure whatever you use has no
significant spikes/ripple and is well behaved on power cycling with no
overshoot when switched on at both light and heavy load. A linear power
supply might be preferred due to lower noise and ripple, but high current
linear power supplies are large, heavy, and are relatively uncommon these days.
And, such a supply may not necessarily be any safer for the laser diode.

Current limiting is provided by 1 or 2, 0.1 ohm 50 to 100 W power resistors and
1 to 4 high efficiency high current series silicon diodes to drop the voltage.
A version of this rig is shown in Quick and Dirty High
Power Laser Diode Driver 1. The diodes have a voltage drop of about 0.5 V
at 20 A. With an appropriate combination of resistors and diodes, a current
from about 5 or 6 A to 30 or 40 A can be selected. A protection circuit (more
for peace of mind than actually likely to do much of anything) consisting of a
0.1 uF capacitor, 100 uF capacitor, 100 ohm resistor, and reverse polarity
prevention diode is connected at the laser diode being tested.

For operation of a few seconds - just enough to make an output power
measurement, active cooling isn't needed for the power supply components
and using the 100 W (or even 50 W) resistors instead of the 250 W dictated
by P=I*I*R at 50 A should be acceptable.

If the laser diode bar or array is already mounted in a massive heatsink,
it too will be fine for 10 or 20 seconds. But if it is just a small
assembly, then cooling will be essential even for this short time. Where
the diode package itself has water cooling lines, it may require flowing
water even if being powered for an instant. If there is any doubt, assume
cooling is essential no matter how short the test.

All connections should be changed ONLY with power off and current at zero.
Even the charge on a 1 uF, 5 V capacitor can damage a 35 WATT, $10,000
laser diode if it is not current limited to a safe current for the diode!
All connections must be very secure using screw terminals or clamps - no
flimsy alligator clip leads! Wiring must be adequately sized (#14 minimum, or
preferably #12 or larger, even for short runs).

Monitor the current by measuring the voltage drop across the power resistor(s).

Leave shorting strap in place (or provide temporary shorting strap if the
original shares stud locations with the main power connections) and attach
power supply cables. Double check polarity and tighten connections securely.
Remove shorting strap.

Attach cooling water lines in correct direction (if marked). Start water
flowing, making sure it goes through the laser diode cooling channels.
Note that in the case of some really high power laser diodes,
what might look like a separate cold plate actually
sends water through channels in the laser diode itself vis a pair of tiny
O-ring sealed connections on its bottom surface.

Make sure a brick, concrete block, or other non-flammable beam stop is
positioned as close to laser output aperture as practical for measurements.

Set up laser power meter or some other means of measuring output power.
It must be capable of handling the maximum possible output power at the
current to be applied to the diode.

Put laser safety goggles on your head in front of your eyes. and the eyes
of anyone else present.

Apply power and ramp up slowly to threshold current (between 15 and 20
AMPs for the diodes described in the next section). Confirm onset of lasing
with IR detector card or other means. Or, set your diode/resistor rig to
the lowest current setting to start.

CAUTION: If power as determined by brightness of glow or meter reading
isn't as high as expected or decreases without reducing current, power
down immediately as it is likely the cooling is inadequate.

Increase current to desired operating level. For an adjustable power
supply or laser diode driver, this can be done directly. For the
diode/resistor rig, it means powering down, waiting for the capacitors
to discharge, changing the jumper(s), and powering up again.

For the types of diodes described in the next section, a red brick
will begin to glow orange or yellow at the beam focus when the power
exceeds 15 or 20 W. Check uniformity of beam

Make the power measurement, record reading and diode ID#.

Powering off/disassembly checklist:

Power down preferably by reducing current to zero and then removing AC
power to the power supply.

After a minute or so, turn off cooling water and disconnect water
connections.

The package is about 15 cm long and shoots a rectangular beam out the window
at the right that focuses to a 1.5 cm line about 15 cm beyond it.
On the sample in the photos, the threshold current is
around 17 AMPS (!!!) and the slope efficiency is about 0.5 or 0.6 W/A. I
could only go to 30 A using my cobbled together power supply described in the
previous section. At this current, it produces 6 or 7 W. The slope
efficiency seems a bit low but perhaps some power is being lost inside
the box or maybe it's just a bit tired after long hours of plate-making.
A similar diode is rated at 35 W max and 65 A`max (whichever comes first)
with a typical threshold of 18 A.

I had assumed the wavelength would be around 830 nm based on
the intended application (see below). However, I have been told that it is
made by Coherent, Inc., and may be closer to 810 nm which could be good for
side-pumping Nd:YAG. Another sample which I tested for wavelength indeed
showed multiple modes between 808 and 813 nm. This might be acceptable
for pumping Nd:YVO4 but probably less than ideal for Nd:YAG
which has a narrower absorption band.

The diode bar is just to the left of the two right-most screws in the
third photo. It's only about 6 mm (1/4 inch) in length and has a cylindrical
microlens for fast axis beam correction glued directly to the diode mount.
Unlike the smaller diode lasers described above, this is not a fiber lens
but a glass plate with a molded cylindrical lens along the top edge.

A few mm away from the diode is an "integrator" or "homogenizer". This
is a plate with a rippled surface (imagine wiggly vertical lines) whose
purpose is to improve the horizontal uniformity of the output beam.
It does this by imaging the diode stripe to focal points just in front of
the plate. The large objective lens assembly placed at a distance of
exactly its focal length away collimates each of the plate's focal points
spreading it over the entire width of the output beam.

Glued to the left side of the integrator is a half wave plate to rotate
the polarization from horizontal to vertical. I assume this is needed for
the intended application.

A roof prism folds the optical path to reduce the size of the diode
package. So the beam path is from the diode shooting to the left, reflected
off the roof prism, and exiting via the objective lenses at the right.

There is a temperature sensor (its two pin connector is just visible
in the third photo) but no TEC. At this power level, water cooling is
about the only option and for this particular diode, it is provided by
a separate water coupling plate which sends water through channels
in the laser diode package itself via a pair of small O-ring-sealed holes
on its bottom surface. Although, the diode seems to survive without
water cooling for a few seconds of testing, this isn't recommended.

Without the integrator plate, waveplate, and objective lenses, most or all
of the beam still exits the laser but it is modestly diverging. How do I
know? Because this laser originally had those components knocked off and
just bouncing around inside the package. While there is some surface
damage to the broken off optics, they are still usable, though probably not
to factory specs. Amazingly, the diode bar itself seems to have survived
despite the original trauma and subsequent shipping.

This laser is probably used in an Agfa platesetter along with a Silicon
Light Machines linear spatial light modulator using "Grating Light Valve" or
GLV technology as they call it. Essentially, the output
of the diode is a rectangular beam that focuses to a 1.5 cm long line about
15 cm beyond the output aperture. The focal point is at the modulator - a
MEMS (Micro Electro-Mechanical System) device that can selectively reflect
or diffract the beam at 256 or more individual locations.

The reflected light from the GLV modulator is reimaged onto a master
printing plate rotated on a drum and thus scanned helically with some
number of pixels written simultaneously. This has some key advantages.
Rather than having a gazillion individual diodes as in systems using the diodes
described above, this uses a single BIG diode laser. The GLV device
provides higher resolution and greater flexibility as well.
And there should be a lot lower cost of maintenance unless, of course,
the BIG diode in the BIG diode laser dies!

The other BIG diode laser which I've tested is part of a mostly complete Agfa
platesetter print engine and includes an 80 AMP power supply. The modulator
is also present, though I have no idea how to control it so I've just tested
the laser and power supply.

The diode laser is in the angled package labeled "LIMO" and is functionally
similar to the BIG gold one but the optical arrangement differs somewhat and
it has the water line connections directly to the diode package. (Some
later versions of the Coherent BIG gold diodes do this as well, see below.)
LIMO is a manufacturer of
many types of high power diode lasers. This exact model doesn't appear
on their Web site though.

The power supply and modulator are also water cooled. For the power supply,
I assume this was just convenient since it doesn't really dissipate that
much power at least on the grand scheme of things and air cooling should
be adequate. The modulator likely requires water cooling because when the
beam at a particular pixel is defracted rather than reflected, it probably
hits and is absorbed inside the GLV device and the total area is very
small. The beam from the LIMO box exits just below the triangular
yellow warning sticker, hits the modulator, and is reflected underneath
through a couple of fairly fancy lenses. One of these is a motor controlled
zoom lens to fine tune the size of the projected pattern onto the printing
plate. Then the beam goes out the aperture in the front, just visible in the
upper left corner of the casting.

The power supply is slick. :) It is a high efficiency switcher programmable
from about 3 A to 80 A via a 0 to 4 VDC control signal with a calibration of
approximately 20 A/V. (It's not possible to shut off the output completely
and the linearity at low current isn't very good. But 3 A is so far below
the lasing threshold that it really doesn't matter.) The actual measured
current is available as another signal, also with a calibration of 20 A/V.

The power input is 180 to 250 VAC, though I suspect that this could be
converted to 90 to 125 VAC with some minor changes. There are a pair of
large main filter capacitors that would be part of the usual doubler
but no obvious jumper for input voltage. Besides the jumper,
the on-board fuses would need to be increased in current rating.

After first confirming the operation using a BIG laser diode simulator
consisting of a pair of high current silicon diodes and a 0.1 ohm 50 W
power resistor (part of my cobbled together high power driver was pressed into
service here!), I powered up the LIMO diode laser. Its lasing threshold is
similar to that of the BIG gold one - between 18 and 20 A. At a current
of 40 A, the output power is around 20 WATTs! A piece of wood placed
in front of the modulator to protect it immediately starts smoking profusely
at this power level and would no doubt burst into flames after a few
seconds. I expect that going to at least 60 A would be safe for the
diodes and should result in over 38 WATTs. The CDRH sticker rating is
50 WATTs so even more power may be possible. :) However, if it's similar
to the BIG gold diode, above, then the rated maximums for power and
current are 35 W and 65 A, respectively.

I tested another sample for wavelength and found it to be around 802 nm,
even further from the 830 nm than expected. It's spectral width was
about 3 nm, somewhat narrower than that of the BIG gold diode, above. This
one might be usable for side-pumping a YAG rod, something I might consider
attempting in the future.

CAUTION: Water cooling is essential for proper operation and to avoid
damage to the diode. Unlike the BIG gold diode laser which seems to be
happy for a few seconds at least without cooling even at reasonably high
current, the output of the LIMO diode laser drops off almost immediately
unless there is flowing water. Apparently, there is very little thermal
mass between the laser diode bar and the water cooling channels. The flow
can be quite low - almost a dribble - but make sure the diode laser is primed
by closing the red valve to the power supply and modulator cooling channels
for a short while to force water through the laser diode channels. Then,
reopen it. Since the plumbing includes rubber tubing, don't let the water
pressure become excessive. There must be a flow restrictor or thermostatic
valve in the diode laser water line since it seemed to significantly restrict
the flow at room temperature. (There is a device with three wires attached
to it but I haven't determined its function. I assume it's either a flow
detection sensor, a temperature sensor, or both.)

By the way, when water leaks inside one of these units, it's not
a pretty sight. I was given one of the BIG gold diodes where this must have
happened. Upon applying power, it was obvious that something was very wrong
as it was drawing at least 15 A at less than 1 V, almost a dead short, and
the current was erratic. And the inside surface of the output window was
fogged! There was also evidence of corrosion on the outside of the case
so I'm not really sure exactly what happened. Maybe the water pressure
regulator failed and the pressure went too high blowing out some O-ring
seals and allowing water to both enter the interior and leak out of the
cooling lines. Or, possibly, the leaks occurred at the O-ring seals as a
result of defective/cracked gold plating/paint. Either way, when I received
the diode, the damage had been done. At least it was probably a quick
painless death for the diode bar. Too gruesome for pics though. :)

Since the types of laser diodes from CD players and other optical storage
devices and laser printers produce IR wavelengths (e.g., 780 nm) and for all
intents and purposes are invisible, some means of sensing their output is
needed for testing. There are a variety of ways of doing this.

A simple IR tester circuit using a photodiode can be easily constructed
from components you probably already have in your junk box. See the
section: IR Detector Circuit.

If you are trying to use a video camera or camcorder as an IR detector,
confirm its sensitivity to near IR by looking at an active IR remote control
through its viewfinder. It may have a built in IR blocking filter which
will prevent it from being sensitive to IR. This may be removable.

The salvaged IR sensor module from a TV or VCR may also be used as an IR
detector. These usually operate from a single supply (12 V typical) and
output a logic signal. However, since these are designed to work with
the modulated IR signals from remote controls and similar devices, they may
not respond reliably or at all to a steady IR output. These can also be
purchased from and electronic distributor and even Radio Shack.

This IR Detector may be used for testing of IR remote controls, CD player
laser diodes, and other low level near IR emitters.

Component values are not critical. Purchase photodiode sensitive to near IR
(750-900 um) or salvage from opto-coupler or photosensor. Dead computer
mice, not the furry kind, usually contain IR sensitive photodiodes. For
convenience, use a 9V battery for power. Even a weak one will work fine.
Construct so that LED does not illuminate the photodiode!

The detected signal may be monitored across the transistor with an
oscilloscope.

Testing of Some Selected Laser Diode and Driver Combinations

Due to the availability of sample devices, I did some experiments with this
combination - which appears to be very nicely matched. The parts values of
the iC-Haus demo board worked perfectly with the Toshiba laser diode.

The Toshiba
TOLD9421 is a typical 650 nm index-guided laser diode with a 5 mW maximum
output. It uses the (now) less common larger 9 mm package. However, that
larger package does provide somewhat more of a built-in heat sink.

The iC-Haus WJB
laser diode driver which supports both CW and pulsed operation up to about
300 kHz with minimal external components. It comes in an SO8 (SMT) package.

The circuit I used is shown in iC-Haus Laser Diode
Driver Test Circuit. This is basically their demo board attached to my
bench power supply (but the simpler one described in the section:
Sam's Laser Diode Test Supply 1
would also have been suitable). For continuous operation, I clamped a power
transistor style heat sink to the laser diode. Without this, the LD current
would increase significantly (by 20 percent or more) within less than a
minute. With the heat sink, there is minimal change.

According to the spec sheet for the TOLD9421 the monitor photodiode (PD)
current can vary from .25 to 1.7 mA (at 5 mW) depending on the particular
device sample. I started with RSET - the resistor that determines feedback
sensitivity - of 50 K ohms and with the function generator disconnected (so
that RMOD wouldn't matter). Based on the transfer function of PD current to
RSET current, this would result in about 72 uA for the actual PD current - well
below the worst case minimum value (at 5 mW) for any sample of the TOLD9421.
Using my variable power supply, I ramped the voltage up gradually to assure
that the device was going to regulate properly - it leveled off at a fixed but
relatively weak output, above threshold but not very bright. After some
trials with lower values of RSET, 15K resulted in an estimated output power of
about 1 mW.

The next step was to try some modulation. Just attaching the function
generator (powered off with its output control all the way down) doubled LD
output since the output impedance of 50 ohms cut the value of RSET nearly in
half (to 7.5K). Then, powering the function generator and cranking up it's
output level allowed me to easily modulate the LD's output between near no
light output (way below threshold) and perhaps 4 mW (still all estimated). I
only tried frequencies I could see with my very accurately calibrated eyeballs
waving from side-to-side - from 0.1 Hz to a 1,000 Hz or so for these initial
experiments.

Modulation works by varying the voltage on the input to RMOD and thus the
current through it from the ISET pin which is maintained at a constant
voltage (about 1.22 V nominal). The PD current is maintained at about 3
times (nominal) of this value.

I could detect no changes in the TOLD9421's behavior (either optical or
electrical) so at least so far none of this has resulted in any detectable
damage to the laser diode. There has been no increase in threshold or
operating current and no measles (spots) in the device's output beam pattern.
(For a couple of minutes I thought there had been damage but the spots turned
out to be dirt on the LD window.)

CAUTION: For experiments like this with a signal or function generator, make
sure that no power or output glitches (as when changing modes) could result
in an excessively negative spike or offset which may force too much current
through the LD and damage or destroy it. The addition of a reverse biased
diode across the modulation input is recommended to prevent excessive negative
voltage from appearing there.

Later, I popped in a
Blue Sky
Research PS106 which is a 7 mW Circulaser(tm) - a 650 nm laser diode with
a built-in microlens to correct for beam asymmetry and reduce divergence.
Since this device had a less sensitive monitor photodiode, I used an RSET of
39K which would run it at about 2.5 mW (I have a printout of this specific
sample's complete electrical and optical characteristics). That worked fine
as well though I didn't puch my luck any further (e.g., boosting power or
modulation). (The PS106 is no longer available but there are now many
other choices on the Blueskyresearch Web site.)

Due to the availability of sample devices, I did some experiments with this
combination - which are designed to work well together probably for laser
pointer and diode laser module applications.

The NVG D660-5 is
a typical 660 nm index guided laser diode with a 5 mW maximum output. It
uses the very cute and compact 5.6 mm package.

The NS102 is an equally
cute and very compact laser diode driver which includes a power adjust pot.
It is designed for use inside laser pointers and compact laser diode modules.
The entire PCB is only 5.4 x 7.8 mm.

The toughest part about testing these was soldering the power supply leads
to the NS102. I totally destroyed the first sample attempting to solder to
what looked like a pad for the positive power supply input but despite its
appearance, solder just wouldn't stick. And in the process, I managed to
lift another pad clear of the device. After a total kludge soldering job
that looked like it should have worked, there must still have been a problem
because upon powering up using my variable voltage power supply with
adjustable current limit, while the regulator appeared to be doing something
based on the brightness of the LD output, power supply current kept going
higher and higher as the input voltage was gradually increased. Eventually,
the laser diode developed those dreaded spots and while still lasing, must have
lost approximately half of its mirror facet(s) as there is also a large dead
area in the beam pattern.

The second attempt was much smoother. Rather than trying to solder to
that pad, the positive connection simply went to the common pin of the laser
diode. So, wiring is as follows:

Positive of power supply: LD can or heat sink or thin wire (e.g., #30
wirewrap wire) to common pin of LD.

Negative of power supply: Thin wire to pad on PCB at far end from LD.

Laser diode: Trim leads to about 2.5 mm (1/8") if desired and form so
they grip PCB resting on the three pads. Just a touch of the soldering iron
will form a nice joint - the pads must be pretinned with low temp solder
and this is sucked up by the gold plated LD leads.

For these laser diodes, the current for 5 mW output is around 27 mA. I used
my variable power supply to assure that the current was limited to 20 mA, then
set the power adjust pot so that the regulator reduced the current. At this
point, I turned up the current limit and finally adjusted the pot for 25 mA
current producing approximately 5 mW output.

I later tested that damaged LD using the iC-Haus WJB driver (see the section:
Testing the Toshiba TOLD9421 with the iC-Haus
WJB Driver, above). It would still operate stably with an output of a
milliwatt or so using optical feedback but about twice the normal current
(50 mA) for 5 mW output. Of course, the unsightly blemishes in the beam
pattern were still there. :( Interestingly, while determining a resistor
value that would work, the current repeatedly spiked to more than 5 times its
specified nominal value (pegging my 100 mA meter) for a good fraction of
second. However, no further damage to the laser diode appears to have
occurred. In fact, output power could still be pushed much higher - perhaps
up to 3 mW or more - but then the current was way off scale and I didn't hang
around to see what would happen next. :) This is in sharp contrast to the
behavior of a laser diode I blew a while back where at a current only slightly
above the rated maximum, the conversion to an expensive LED was quite rapid.

This combination is designed to fit entirely inside NVG's machined brass
Laser Diode Module Housing
which provides the much needed heat sink (the laser diode current would begin
to creep up almost immediately due to the small thermal mass of the 5.6 mm
laser diode package) and an adjustable collimating/focusing lens. Once
assembled, the commercial units are potted in Epoxy and the laser safety
sticker is wrapped around the outside. :)

The iC-Haus IC-WK laser diode driver is intended for CW and low frequency
modulated operation with a 2.4 to 6 VDC power supply. (See:
iC-Haus for detailed information,
under "Laser Drivers".)

I soldered another NVG D660-5 to the iC-Haus IC-WK demo board (WK2D). The
WK2D can be used inside a laser pointer though not quite as small as the
NVG driver board described above. The WK2D is intended for laser diodes
where the COM lead is the anode of the LD and the cathode of the PD (most
common type). The IC-WK driver can be configured for any style of laser
diode package. (There is also a WK1D demo board for laser diodes with
common LD/PD cathode and with common LD cathode/PD anode pin configurations.)
And, in conjunction with an external transistor or MOSFET, can be used with
higher current laser diodes as well.

It took about 2 minutes to solder the power supply wires and laser diode.
Thankfully, although the circuit board is fairly small, nice tinned solder
pads are present and soldering was a snap. :)

For my initial testing, I used the adjustable power supply described above.
I brought up the voltage just to the point where there was some output from
the laser diode and adjusted the pot until the driver started regulating.
Had I just switched on power within the driver's rated voltage range, it's
quite possible the laser diode would not have been happy. Later, I replaced
the bench power supply with a pair of AA Alkaline cells which at 3 V, is well
above the 2.4 V required by this cute little driver.

The usable range of monitor photodiode current over the adjustment range
for the WK2D as configured is about 35 to 100 uA. I realized later that
the monitor current for the D660-5 is only about half of the minimum required
for the WK2D to regulate so my poor little 5 mW diode was actually running at
about 10 mW. The first one actually survived and would operate at this
output power continuously. However, adjusting the pot to anything but the
highest value eventually resulted in its demise and some other samples weren't
as robust.

Use of Salvaged CD Laser Diodes, Substitution

While your first instinct may be to rip the laser diode out of its original
mounting, this is often unnecessary and undesirable. Depending on your
application, using all or part of the assembly may simplify positioning and
control of the laser beam.

For CD and other optical drives, the optical block (often called the
optical pickup) includes the laser diode, various optics, objective lens
mounted on two-axis actuator (focus and tracking), and the photodiode array
for servo control and data read-back.

Note: Some designs combine the laser diode and photodiode into a single
package which is then mounted in the optical block. This can still be used
for either or both functions as long as you can identify the proper pins.

For laser printers, the optical block will include the laser diode and
collimating/focusing lens (and possibly some other optical elements).

In some higher performance printers, there may be a Peltier cooler attacted
to the back plate of the laser diode. Pretty cool :-) (no pun....).

Some laser diode power control and protection components may also be present.

Note: There are often a pair of adjacent solder pads connected to the laser
diode circuitry on the flex cable or circuit board associated with the optical
block. When handling the assembly but not actually attempting to power the
laser diode, it is a good idea to short these together with a drop of solder
using a grounded soldering iron. This will prevent the possibility of ESD
damaging the laser diode.

Where the laser diode is to be used as part of a precise optical apparatus
for close range sensing or scanning, for example, the entire optical deck
(including the stable mounting and sled drive mechanism) may be useful intact.
For the typical three-beam pickup (most common), this will provide precise
control of beam position: Y (focus), X-coarse (sled drive), X-fine (tracking).

There are several good reasons to leave your CD laser diode installed in the
optical block assembly even if you are not going to use it with the objective
lens and focus and tracking actuators which were part of the pickup:

The metal casting provides the very important heat sink which is necessary
for continuous operation. Not all optical blocks are made of metal but for
those that are, the cooling function could be important.

There is less risk of damaging the laser diode through careless handling and
ESD.

There may be a collimator lens in there - probably the first or second
optical element in front of the laser diode. It may be combined with the
laser diode in its metal barrel. If there is a collimator, you should be
able to get a nice nearly parallel beam without much work. At most, a
small lens will be needed to optimize it.

Remove the objective (front) lens and its associated coils unless you
require them for a short range application. They will likely come off as
a unit without too much effort. However, try not to destroy this assembly
as you never can tell what might be needed in the future.

The multisegment photodiode sensor and focus and tracking actuators may be
useful for a variety of applications. Think twice before ripping it apart
if you require any of the capabilities originally present!

While there are many variations on the construction of optical pickups even
from the same manufacturer, they all need to perform the same functions so the
internal components are usually quite similar.

The laser diode assembly and photodiode chip connections are typically all on
a single flex cable with 10 to 12 conductors. The actuator connections may
also be included or on a separate 4 conductor flex cable. The signals may
be identified on the circuit board to which they attach with designations
similar to those shown above. The signals A,C and B,D are usually shorted
together near the connector as they are always used in pairs. The laser
current test point, if present, will be near the connections for the laser
diode assembly.

It is usually possible to identify most of these connections with a strong
light and magnifying glass - an patience - by tracing back from the components
on the optical block. The locations of the laser diode assembly and photodiode
array chip are usually easily identified. Some regulation and/or protection
components may also be present.

Note: There are often a pair of solder pads on two adjacent traces. These
can be shorted with a glob of solder (use a grounded soldering iron!) which
will protect the laser diode from ESD or other damage during handling and
testing. This added precaution probably isn't needed but will not hurt. If
these pads are shorted, then there is little risk of damaging the laser diode
and a multimeter (but do not use a VOM on the X1 ohms range if it has one) can
be safely used to identify other component connections and polarity.

Specs of laser diodes with similar wavelengths vary quite a bit, especially
the monitor photodiode current sensitivity. You can't just drop in any old
laser diode that fits and expect it to work even if the pinout is the same.
It might indeed work, it might be too dim, or it might blow out. There is
a good chance of the latter. The only way to be sure is either to analyze the
circuit to know what its compliance range (drive current and feedback current)
is or to determine the actual specs of the original laser diode. Only then
can a suitable substitute be selected. Another alternative is to make changes
in the driver circuit to handle an available replacement. Note that for
CD, DVD, and other similar applications, even an exact replacement may not
work without precise optical and electronic alignment since the physical
position and orientation of the laser diode chip, as well as its precise
output power, may be critical. Also see the next section.

While the small laser diodes we are dealing with are similar in many ways,
there are enough differences such that substituting one for another is not
trivial. The problems are fourfold (at least!):

The package type and size may differ. The new one may not fit properly!

The pin configuration and polarities of the laser diode and the monitor
photodiode may differ. The latter, in particular, could require substantial
modifications or total redesign of the driver circuitry.

The driver circuitry will need to be modified for the different electrical
characteristics of the replacement laser diode.

The required current will be different. For example, it is probably
lower for an IR laser diode than for a visible one.

The monitor photodiode sensitivity will be different.

If you were to just pop in an IR laser diode in place of a visible one,
either it will not work at anywhere near maximum output and/or it may blow
instantly.

Where the wavelengths differ substantially (e.g., 780 nm vs. 670 nm) the
optics may no longer focus or collimate properly. With luck, there will be
enough of an adjustment range - if the optics are not totally sealed and
glued in place!

This can probably be done but expect to blow some laser diodes if you are not
extremely careful - and even perhaps if you are!

Should the optical window on a metal laser diode package become damaged or
broken, it may be possible to remove the entire cover. I don't recommend
attempting to break out the window for fear of damaging the actual laser
diode chip just behind it. Rather, take a triangular jeweler's file and
make a groove as close to the base as possible all the way around, going
just deep enough to make it through the outer case. The entire cover will
then pop off. Securely SHORT the leads of the laser diode together to
prevent ESD damage as you do this. While the exposed laser diode chip
won't be as protected as inside the can, with care it will survive
especially if some substitute means of keeping out environmental
contaminants is provided.

Laser Diode Life, Damage Mechanisms, COD & ASE, Drive, Cooling

For all intents and purposes, low power laser diodes in properly designed
circuits do not degrade significantly during thousands of hours of use or when
powered on or off. However, it doesn't take much to blow them (see the
sections: Low Power Visible Laser Diodes and
CD Player and Other Low Power IR Laser
Diodes). I have seen CD players go more than 10,000 hours with no
noticeable change in performance. This doesn't necessarily mean that the
laser diode itself isn't gradually degrading in some way - just that the
automatic power control is still able to compensate fully. However, this is a
lower bound on possible laser diode life span.

Some datasheets list expected lifetimes for laser diodes exceeding 100,000
hours - over 12 years of continuous operation. Of course, I trust these
about as much as the latest disk drive MTBFs of 1 million hours. :-)

Laser diodes that fail prematurely were either defective to begin with or,
their driver circuitry was inadequate, or they experience some 'event'
resulting in momentary (greater than a few microseconds) overcurrent. What
this means is that with cheap driver electronics such as found in many laser
pointers, leaving the thing on continuously may result in much longer life
than repeatedly pulsing it.

As noted elsewhere, a weak laser diode is well down on the list of likely
causes for CD, LD, MD, and DVD player, as well as laser printer problems.

High power laser diodes may have considerably shorter life expectancies than
the 5 mW variety - 10,000 hours or less.

And, high temperature operation can reduce life expectancy, possibly by as
much as a factor of 2 for each 10 °C rise above the temperature quoted
in the device's specifications. Thus, a laser diode with a quoted life of
10,000 hours at 25 °C, might only last 125 hours at 55 °C. Not
that it will actually fail at 125 hours and 1 second, but its maximum output
power will be reduced by 50 percent. I expect that there is a wide variation
on the extent to which this applies depending on device type, how close it is
operated to its specified maximum power, and all sorts of other factors.

Of course, in the grand scheme of things, even LEDs gradually lose brightness
with use.

(From: Gregory J. Whaley (gwhaley@tiny.net).)

There is one thing to keep in mind about laser diode lifetimes. The time to
failure probability distribution is quite wide, meaning that some laser's
lifetime will be significantly less than the 5,000 hour mean, and some will be
much, much longer than the mean. Lasers are not like light bulbs where they
"wear out" and have a predictable lifetime. The main life limiting factors in
a laser diode are related to how many crystal defects are present in the
device when it is made. If you are lucky to have a diode with very few
defects, then your laser may last nearly forever. If you are not so lucky, it
may only last a few hours.

Overdrive or other abuse of a laser diode may result in total destruction and
instant conversion to a DELD (Dark Emitting Laser Diode). However, what is
more likely to happen is that the device will either still produce some
coherent output (but at reduced power levels) or turn into an expensive LED.

Assuming the device was operating above its threshold current with a nice
bright output beam prior to the 'event', some or all of the following may
be in evidence:

The output intensity is somewhat reduced at the same current level - The
laser diode has likely been damaged but not totally destroyed. It may still
be usable but will no longer be able to produce its full rated output power.
However, its beam profile will likely have suffered (see below). Don't be
tempted to boost the laser diode current to obtain the same output power as
before. You will likely cause further damage and possibly complete its
conversion to an expensive LED.

If you return a damaged laser diode to a driver that uses optical feedback
to stabilize output power, the laser diode will likely be destroyed if the
circuit increases the drive current to its maximum limit in a futile attempt
to achieve the expected output power.

The output intensity is much much lower at the same current - You have an
expensive LED. Note that the lack of coherent light will not be instantly
obvious from the optical properties of the output beam. You will still be
able to collimate or focus it quite nicely compared to an LED because the
emitting area is much smaller than an LED - perhaps as little as 1 x 3 um
for a 5 mW laser diode compared to around 250 x 250 um for a typical LED.
However, the intensity of a functioning laser diode has the characteristic
wedge shaped output pattern while that of the resulting LED is more diffuse.
Thus, viewing the output will result in a distinct peak close to the optical
axis if it is still a laser diode. (Of course, I assume this viewing
is being done safely!)

Beam intensity doesn't increase dramatically as the current is raised (as
it would with the positive feedback of an intact laser cavity) and there
will be no distinct threshold; output will be pretty much linear with
respect to current.

The output power doesn't change monotonically with current - This is
particularly evident on higher power laser diodes that have been
traumatized. This may be due to a variety of damage mechanisms including
(1) that the preferred transverse mode structure changes with increasing
current, the damaged areas of the facets (mirrors) will interfere with
the smooth increase in output power and (2) impurity migration or other
defects in the junction due to excessive current. There will be certain
current levels where the output power will dip a bit, decreasing when the
current is increased.

The beam characteristics have changed - A damaged mirror will likely
result in all sorts of effects on the beam even if the device still lases.
It may cut off part of the beam changing its shape, symmetry, or uniformity;
act like a slit or diffraction grating and produce side-lobes, or any of a
number of phenomena resulting in unsightly blemishes that can only be
described with photos of the beam profile (more below). Slight damage may
result in what I like to call "measles" - a few dark spots at random
locations. However, first clean the laser diode's window - I have been
misled into thinking damage had occurred when in fact only some specs of
dust had decided to land on the window!

For the typical low power laser diode (e.g., NVG D660-5), a common effect
is for the normally nice smooth elliptical beam to develop dark stripes
parallel with the fast axis corresponding to damaged sections of the facet.
With my experiments (some semi-intentional, others accidental), they were
more or less symmetric on either side of the center of the beam.
Interestingly, on a few samples, some degree of this effect was totally
reversible when current was reduced indicating that actual damage hadn't
yet taken place. On one in particular, it was possible to run at a total
output power (every photon captured by my power meter) of over 15 mW (keep
in mind that these are rated 5 mW max) but after a few seconds, the banding
would start appearing. Killing power and letting the device cool then
restored the normal beam pattern. At an output of 10 mW, it could run all
day without problems. At some output above 15 mW, the banding occurred
instantly and was permanent. (There was no heatsink on this device for
any of these tests).

For high power laser diodes such as the type used to pump solid state lasers,
the location of facet damage be even more clearly seen in the beam pattern.
Since the emitting aperture of these may be 100 um or more, projecting the
output onto a white screen using a short focal length lens (e.g., one from
a CD player) will yield the distribution of lasing along the aperture. Set
up the distance between the lens and screen to be about 40 mm. This will
require an LD-to-lens distance of a few mm (for the CD player lens of 4 mm
focal length). The projection will then be a line 2 or 3 mm in length. A
new/good LD will have a smooth and nearly constant brightness (if visible or
through an IR viewer) but a damaged one will have significant variations
in brightness as well as places where there is no light at all. A common
failure characteristic is to just have the side lobes with nothing in the
middle. However, this terminal disease would also be obvious in the
unfocussed beam pattern. Such serious damage may even be readily apparent
as different color/rough areas on the end facet using a magnifier or low
power microscope.

For some diodes/types of damage, these effects can be quite dramatic and also
violate our belief in instantaneous and permanent damage mechanisms with
respect to laser diodes. One of my NVG D660-5 laser diodes (5 mW max) was
subjected to an overcurrent event which resulted in total loss of regulation
by its driver (perhaps the rear facet was damaged reducing optical power to
the monitor photodiode). The usual outcome of such a failure would be a
totally fried laser diode. However, with this sample, the beam pattern
fluctuated wildly as current was increased from threshold with side-lobes
appearing and disappearing and changing position, with the intensity of the
beam diminishing and finally vanishing entirely. However, this was all
totally reversible by simply reducing the current! At one particular current,
the output looked approximately normal with an output power of 10 mW - twice
the diode's rating In short, even after being subject to such abuse, this
tough diode still exceeded its original specs! It finally succumbed to
further COD (Catastrophic Optical Damage) when switched on at too high a
current after cooling down and produced even stranger beam patterns but less
maximum power. Then, it died completely, turning into a 39 ohm resistor. :(

(Portions from Flavio Spedalieri (fspedalieri@nightlase.com.au).)

A way to determine if a laser diode is damaged is by shining the uncollimated
beam on a white screen and looking at the spread of light intensity - the
beam profile.

This method works with all laser diodes where the light is visible (up to a
wavelength of about 800 nm), or with a CCD camera or other sensor array,
further into the IR - or UV (wishful thinking).

A working laser diode, will produce an elliptical beam, that is brightest in
the longitudinal axis, and tapers off in brightness towards the edges. Some
may have slight bumps or dips or hints of an interference pattern but their
location will usually be relatively symmetric - if one of these features
occurs on one side, there will be a similar one on the other.

If you drive a diode at even very slightly above its maximum limit, you will
cause permanent damage to the diode over time.

If you take a diode, then drive it with the correct current, the above beam
profile will be produced. If you begin to slowly increase current, up to a
certain point, the optical output will increase. Continuing to increase the
current beyond this upper limit, the appearance of the beam will begin to
change, the output will start to decrease, then the beam will have light and
dark bands through it - the diode junction and/or mirror facets have now been
damaged.

At this point, the diode is still producing coherent radiation, with slightly
reduced output power. If you try and collimated this beam, you will end up
with a spot that has light and dark areas.

This type of damage is caused by exceeding the limits of the structure of the
semiconductor material and is irreversible.

When asked the question: "How sensitive are laser diodes to drive and
handling?", there will likely be a variety of responses from either side:

"I just connected a bare laser diode to an automobile battery without any
other components and it is working just fine. I have never used any ESD
precautions. In fact, I have a wool sweater on at this moment and can draw
some really juicy sparks from everything I touch."

through:

"I have blown several hundred laser diodes and I have been following all the
manufacturer's guidelines with respect to ESD protection and drive. I am
even using their recommended circuit layout and $4,000 power supplies.
Nothing seems to help."

Not all laser diodes are created equal and their susceptibility to damage
through improper handling or improper drive likely varies widely. Here is
a discussion of some of the issues:

(From: Eric Rechner (eric_r@3dm.com).)

"Does anyone have any experience with Hitachi laser diode HL7843MG 5 mW 780nm?
I find this diode to be possibly extremely sensitive (ESD??), more so than
any other 780nm laser diode. Does anyone know if there are problems with
Hitachi MQW type diodes? Are MQW diodes more sensitive to ESD than Double
Heterojunction diodes? Does anyone have info on possibly 'bad' or defective
lasers out there?"

(From: Jon Elson (jmelson@artsci.wustl.edu).)

Strange. I think I've used some of these.

I hear everybody babbling about extreme static sensitivity on these devices,
yet I've never had a failure, and I've been using just the usual minimum
precautions with any semiconductor device. I suspect that people may be
exceeding the optical power MAXIMUMS on the devices. I've been very
conservative on that, since the devices only carry an optical maximum, and
don't have that correlated to forward diode current (difficult, because it
varies strongly with temperature). I try to run them at a good bit less than
rated power, maybe 2 to 3 mW optical output. I'm using a diode sold by
Digi-Key for $19.00, just because it is cheaper than the Panasonic in the
5.4 mm case. I think the manufacturer is NVG or something like that. I've
got 10 of them I am working with, designing a closed-loop driver for a
photoplotter, which pulses the lasers on and off as fast as 10 us on, 10 us
off. It is working pretty well now. I included a series resistor (as well
as the control transistor), so that if the loop becomes unstable or the
sensing diode gets disconnected, it won't fry the laser diode.

(From: Dr. Mark W. Lund (lundm@xray.byu.edu).)

The babbling starts here: You don't have to be a total idiot to blow these
things, in fact I have blown a few myself. Identifying the source of the
trouble is extremely costly and difficult because it only takes a spike of a
few nS to to the damage. I would say that 99.9999% of the time it is the
power supply. Either it spikes on turn-on, turn-off, or at random. We used
to toast lasers with a $5,000 laser diode power supply that would spike every
time you sent certain signals on the IEEE 488 control line. This was a tough
one to figure out, I can tell you. In the process we tried to damage one
using static to try to get a handle on the sensitivity, but were not able to
get a catastrophic failure this way (we may have induced some latent failures,
however). Other laser diodes may vary.

(From: Jon Elson (jmelson@artsci.wustl.edu).)

Ah! This is good anecdotal evidence! I've often suspected that there might
be more of this going on, and instead of examining the drivers, people just
attribute problems to an invisible gremlin! I sure can see how a closed
circuit driver can oscillate or overshoot on transients, and there could be a
situation where some percentage of drivers will be less stable due to
component tolerances. Unless you rigorously test a good batch of your
drivers, you could have this sort of thing and not know it. (Of course, any
time you put a computer in the loop, especially one that is canned inside
an instrument, then the probability of unanticipated gremlins increases
dramatically!).

Of course, I was designing a fixed-purpose driver to be used in a specific
application, inside an instrument, so I had it easier than the guys designing
a lab-quality pulser for who knows what application. So, I could put in a
resistor, which will limit current to some 'safe' level, even if the loop is
unstable, which it certainly was when I was tuning up my driver.

I DO use generally sound anti-static precautions, almost subconsciously, to
protect all semiconductor devices. But, I am aware that I have occasionally,
by accident, touched a cable going to the laser diode before I was grounded,
and I have never noted a catastrophic failure.

I will have to go through some rigorous life-testing to make sure I'm not
causing latent failures, but I've run these diodes for quite a few hours while
testing things, and nothing of note has turned up yet.

By babbling, I meant some items in print media, as well as a lot on this and
other newsgroups, indicating that if you even touch one lead of a diode laser,
it is ABSOLUTELY destroyed, with a probability of 1.000! Obviously not true!
Your comments are well reasoned, and indicate real experience. Others have
also written that only a huge corporation, with millions in test equipment,
could ever make their own laser diode driver. Now, clearly, the nanosecond
multi-watt pulsers ARE much more difficult to do right, fast risetimes without
overshoot is tricky. But, I did it in my basement with just over $1,000 in
test equipment, mostly a decent oscilloscope. I also had the confidence that
if I DID blow a few diodes, it wasn't so painful at $19 each.

So, now, I'm babbling!

(From: Eric Rechner (eric_r@3dm.com).)

Just an update on the outcome of my question about Hitachi laser diodes,
above. At that time, large numbers of the diodes in question
were dying prematurely (we were running at about 80% full power at a
temperature between 20 and 30 °C, CW for several weeks in
triangulation sensors). Our diode module supplier had the facilities to
inspect the laser chips using electron microscopy and apparently found
that new diodes exhibited oxidation on the facet. They believed this to
be a process problem (contamination) at the manufacturer end. The last I
heard, the diode module supplier credited us with replacement lasers - there
were about 1000 pieces, but this took a great deal of 'fighting'....

A variety of effects are responsible for laser diode failure. The one that
most people are most fearful of is Catastrophic Optical Damage (COD) to the
end facets due to excessive optical power density through them. This is not
just simple overheating as with an underrated resistor but a complex process
that can take place on a very short time scale.

With the active area of the end-facets of some laser diode being as small as
1 x 3 um, it isn't surprising that a little too much power will kill it. The
power density of 5 mW through that aperture is 1,666,666,666 W/square meter or
167 kW/cm2! Apparently some types of optical materials when
properly processed and undamaged can handle more than this without a problem
but GaAlAs or whatever of the laser diode's mirrors isn't one of them.
(Some manufacturers specify the emitting aperture of their laser diodes to be
much larger - 10 x 60 mm being a typical value. However, these dimensions are
inconsistent with their beam divergence which is similar to that of the much
smaller aperture. If the actual emitter were that large, power density would
drop by a factor of 200 and it would seem that COD would not be a major
concern at the same power level.)

However, overall thermal damage is also possible even - or especially - with a
laser diode driver using optical feedback. When you turn up the power control,
there may initially be higher output. But as the laser diode heats up over
a few seconds or minutes, its output with respect to current decreases and the
regulator will keep increasing the current to compensate - potentially a
runaway condition which can also result in damage or death to the laser diode.
A large heat sink, active (e.g., Peltier or heat-pipe) cooling, or dunking in
liquid nitrogen may help if you are really determined to get every last photon
from your laser diode! :)

Or, where the laser diode is powered from a constant current source and set for
a higher output when warmed up, it may blow instantly the next time it is
turned on after having been off for a while. The reason: For the same current,
the laser diode's optical output is greater when cold and may exceed the COD
limits of the its end facets.

In other words, there are many interesting and creative ways to convert a
laser diode into a DELD or expensive LED!

(From: Gregory J. Whaley (gwhaley@tiny.net).)

I will assume the effect is Catastrophic Optical Damage (COD) of the facet.
This is an interaction between the temperature of the facet and its optical
absorption. When the temperature of the facet grows, the absorption can also
grow which feeds back positively to the temperature and the temperature "runs
away" until it is physically damaged. My understanding is that this is
extremely fast, certainly less than a microsecond, probably less than a
nanosecond. COD is often cited as the mechanism which makes laser diodes
extremely ESD sensitive and the ESD discharges can be quite brief.

Optical damage in a laser diode is a fairly complex phenomenon so it is hard
to give time and/or power to damage. But based on my experience I'll give
some numbers.

Typical 5 mW telecom laser diodes (1300 or 1550 nm) are really underated as
far as optical power goes and they in general can be driven at 2 to 3 times
their rated power without any immediate damage though their lifetime may be
months instead of tens of years. High power diodes (e.g., 1 W) on the other
hand are rated near their maximum optical power. How much higher they can be
driven is a function of pulse width and duty cycle. To give some typical
numbers at a pulse width of 1 ms and duty cycles of a few percent: A diode may
be driven at up to 50 percent higher and at pulse width of about 50 ns; at a
duty cycle of 0.1% it may driven at up to 5 - 10 times the rated power.

A diode that has suffered COD is already dead so its ESD sensitivity is a moot
point. On the other hand a diode that has been overstressed optically is more
ESD sensitive. This effect works in reverse too, i.e., a diode that has
undergone an ESD discharge may only be able to handle lower optical power.

I don't think a time for optical damage can be stated without knowing the
stress conditions and the type of diodes. A diode stressed at 20 to 50% may
not suffer any catastrophic damage at all but just die out gradually - just
much faster than normal lifetimes. At about 100% overstress, degradation can
be catastrophic, and fairly fast. Even then the diode can generally be
operated at the higher powers for quite a while (seconds) before the onset of
COD. Once the COD starts it probably is quite brief. I'm not sure about the
numbers and figures mentioned (nano - microseconds) may be correct for actual
COD to occur.

ASE usually stands for Amplified Spontaneous Emission. It is part of any
lasing process, and is just what it sounds like - spontaneous emission (not in
the lasing mode) that gets amplified by the gain medium in the cavity. I find
it easiest to think of this in terms of phase: The lasing mode will have one
well-defined phase, while all the noise (ASE) modes will have some phase shift
relative to the lasing mode. ASE is mostly a concern when you are trying to
send modulated signals (e.g. bits) with your laser diode. In that case, ASE
is essentially a noise source which degrades the signal (or S/N). In most
electrically-pumped diodes, ASE is not so much a problem as RIN (Relative
Intensity Noise), which can raise the bit error rate by changing the relative
levels of the "on" bits.

L-I characteristic for ASE is going to follow the lasing mode for the low part
of the current range, but at some point (depending on cavity Q and carrier
lifetime), you're going to get spontaneous emission clamping, where the ASE
will stop increasing superlinearly. I'm not sure that this is the same as
COD, where you should see a sharp decrease in optical power output.

There are a number of good laser physics books which may discuss this - try
Sargent, Scully and Lamb ("Laser Physics") or Yariv ("Quantum Electronics").

If you intend to use the laser without the feedback, one has to realize that
there are a number of problems. One is that as the temperature goes down, the
laser efficiency goes up. This tends to cause the laser diode to destroy itself
at lower temperatures while running that same current that was OK at some
higher temperature. Generally, if the temperature doesn't vary to much, one
can use something as simple as a limiting resistor and not run the laser at its
highest output. I once made a burn-in driver for some power lasers that used
constant current sources that had no feed back but I had to preheat the diodes
to 100 °C before using that high a level of current. The level of
current used would have wiped the diodes out at room temperatures.

The hardest part of the whole thing was making the circuit to have controlled
levels of current during power on and power off. Most things like op-amps are
not specified under these conditions. My first attempt wiped out 10 diodes :-(
when I turned the power on.

To run the diodes at there maximum light out safely, requires using the
feedback photo diode.

This will determine the maximum frequency at which closed loop optical
feedback can be used for laser diode modulation as well as the minimum
filtering requirements for CW driver circuits.

Note that the photodiode is NOT part of the laser diode structure - it sits
behind the laser diode in the typical package. So, you can actually test its
frequency response with an external modulated light source (like an LED or
another laser diode driven by a high speed pulse generator) independent of the
laser diode itself. The light doesn't have to pass through the laser diode.
Although not terribly clear, the photodiode can be see in the
Closeup of a Typical Laser Diode.

(From: Richard Schmitz (optima-prec@postoffice.worldnet.att.net).)

The frequency response of the photo diode (PIN diode) is usually shown in
the back of the manufacturers laser diode data book. In the case of
Toshiba's visible diodes, the freq. response is shown as flat out to about
10 MHz and it rolls off to -3dB at about 175 MHz. With the newer diodes used
in the DVD products, the freq. response seems to be a little better, curves
for the TOLD9441 show the response out to 1 GHz, down -3dB. If you need
exact details, contact a distributor and get the latest Toshiba data sheets.

Cooling a laser diode will have obvious physical effects like shortening of
the cavity - so mode hopping would be expected. However, there will also be
changes in wavelength, and efficiency will increase. But going to far may
cause structural damage. The efficiency will also increase - to a point -
as the temperature decreases. What this means is that with a constant current
driver, the output will increase as well. However, the limiting factor before
the LD changes into a DELD may still be Catastrophic Optical Damage (COD) and
its onset will depend on the E/M field interaction at the output facet,
something not affected very much by ambient temperature. So, your 5 mW LD
may still be limited to 5 mW even if it is more efficient at low temperature.

"I have read that cooling semiconductor laser diodes shortens wavelength and
greatly increases efficiency some. Does this apply to the 635 nm diodes and
what would be the result of super cooling one of these diodes?"

(From: Fred Kung (kung@ccf.nrl.navy.mil).)

One thing you will need to be careful about is that in super cooling a
compound semiconductor diode laser, you will eventually take it out of its
range of lasing operation (due to dispersion shifting). Dropping the
temperature to -50 °C or so is OK, but don't expect them to work in LN2 or
anything very cold unless they're designed for that.

The 0.3 nm/°C figure is good for GaAs quantum well lasers with AlGaAs
cladding (which covers most of the commercially available ones), but only
around room temperature.

One other thing that may happen if you cool the diode too far is that the
thermal mismatch with the epoxy will cause it to physically come loose from
its mount. Again, a TE cooler is fine, but don't dump cryogens on the thing.

(From: Steve Roberts (osteven@akrobiz.com).)

As diode temperature goes down, so does the level of the damage
threshold.

A friend who makes his living selling OEM laser display systems did
some tests a while back, massive amounts of Peltier cooling (30 to 40 °C)
results in a much lower current for the destructive failure of the
diode, He was blowing off the front faucets of the diodes at less then
normal operating currents. So yes you can shorten the wavelength
somewhat, but you have to test carefully and derate the max current.
Derating the current means less output power, so you probably want to
start with a 40 mW or bigger diode. Basically the intracavity
flux goes way up and often the faucet can't take the increased power
density.

We did some experiments to see whether the types of laser diodes found
in red laser pointers could be pulsed without damage. It seems that
depending on the type of laser diode, pulsed operation in the nanosecond
range may be possible.

A microsecond is much to long for CW diodes, but you can try 10 to 50 ns.
This can work, but it still depends on the laser diode. We performed
experiments with low cost 5 mW, 650 nm CW laser diodes (red laserpointer)
with 50 ns, 3 A, 1 kHz, and the LDs worked without pain (no degradation) for
months. 100 to 200 ns seems to be the critical pulse length. Also the
effective emitting aperture size is important, a 400 mW LD may have a
typical 100 um aperture - compared to a red pointer diode of typical 3 to 5 um.
The power density mW/aperture size is the most critical value, normally you
cannot go much higher than 10MW/cm2 to 30MW/cm2 (Megawatt).
Higher power density at the outcoupling facet means sublimation of "mirror"
material. But don't worry, worst case you have made a EELED...

We made a fast and dirty setup and did not care much about power linearity
by drive current. But laser power was more or less linear and proportional
with increasing pulse current - surely running over some kinks, but this did
not matter in this case. Also some LDs "gave up" catastrophic - as
expected(!!!) - at much lower pulse currents in the 100..200 mA region.

We applied current pulses (fp~10..100 Hz) up to 6 A, typ. 50 ns, but
recognized a fast degradation and EELED metamorphosis within few minutes to
hours of running.

These LDs had PDs inside, TO-18 with window, driver circuit was APC type. But COB
(Chip On Board, bare chip) LD with 50 Ohms "driver" may also work...

The big surprise for me finally was to get out "extremely high power laser
pulses" from a lowest cost red pointer laser diode. Even if you pulse such a
LD at "snugly" 500 mA the pulse power is very high compared to a typical 5 mW
to 50 mA CW current. One last thing: Normally you cannot predict if a CW LD "test
candidate" will survive - it's a real game of trial and error...

Laser Diode Wavelengths, Spectra and Visibility of NIR Laser Diodes

The first direct injection laser diodes (i.e., electrically pumped monolithic
semiconductors), developed in the 1960s at the beginning of the Laser Age, were
pulsed deviced emitting at near-IR wavelengths (and possibly only with
cryogenic cooling), around 750 to 800 nm. As the technology has matured, room
temperature CW laser diodes have become readily available and the range of
wavelengths has expanded to include visible red (670 nm), orange-red (635
to 650 nm), and pushed further into the IR (up to about 2 um). Most of these
are based on various compounds containing gallium and arsenic. To get an
idea of the wavelengths and output powers available commercially, see:
K3PGP's
Laser Diode Specifications maintained by K3PGP (Email: k3pgp@qsl.net).

The use of laser diodes in all sorts of mass produced products (CD, LD, MD,
DVD, laser printers, bar code scanners, telecommunications, etc.) has driven
down prices for lower power devices, at least.

Mid-IR (3 to 25 um) types are also available. These typically use
lead salts for the active material, but may require a frigid operating
environment while producing only around 100 uW output power. You won't find
such devices in consumer electronics - their applications are more likely to
be in spectroscopy research. (check out:
Laser Components GmbH).

(Portions from: Anthony Cook (a.l.cook@larc.nasa.gov).)

The latest development in far-IR (greater than 3 um) laser diodes is the
Quantum Cascade Laser which can produce 100s of mW of light at room temperature
and up to a watt or more when cooled to about -100 °F (-73 °C). These
operate in the range of 3 to 13 um. They are not commercially available yet
(I don't think) but several research groups are doing work in this area:

Some nominally IR wavelengths are indeed very slightly visible. In favorable
conditions (mainly isolating from more visible wavelengths) I have seen with
my own eyes:

The 766.49/769.9 nM potassium lines, as a contaminant in high pressure
sodium lamps.

The 818.3/819.5 nM sodium lines in the spectra of high pressure sodium
lamps.

The 762.1, 759.4, and 822.85 nM earth atmospheric absorption lines in the
solar spectrum. (Usually with the sun somewhat low.)

The output of a laser diode in my CD player is visible at eye-safe
intensities (half a meter from a source with a beam covering nearly a
steradian for a few seconds). I have seen the spectrum of this along with
that of a neon lamp placed next to it, and verified that what I saw was
the laser line, with a wavelength around 800 nM. It could be as low as
around 780 nM.

According to the C.I.E. "Y" or visibility function (or extrapolation thereof),
the visibility of these lines is impressively low. However, considering the
wide dynamic range of the human eye, these wavelengths are visible at eye-safe
levels.

CAUTION: there is no advance warning of having exceeded eye-safe exposure to
slightly visible wavelengths normally considered IR. You may permanently
toast part of your retinas duplicating the above unless you verify retinal
exposure below the Class I laser exposure limit.

I recently got a laser pointer with a wavelength of 660-661 nm or so and
(guesstimated) 2 mW of output power.

I discovered that if I shine the beam through one of those dielectric
interference bandpass filters, I got some weak beam output at other
wavelengths. So, I investigated further.

About (very roughly estimated from standard issue eyeballs) .2 percent of the
beam is spurious radiation with a continuous spectrum. I don't yet know well
what it does at longer wavelengths, but a majority of the short wavelength
side of this is in the few tens of nm below 660 nm. Slight traces exist down
to 540 nm. With two 532 nm filters, I could stare into the beam and see a dim
point of light. With a 570 nm filter, it was slightly bright to stare into
and I could see the beam VERY DIMLY on a wall in a dark room. With a filter
around 630 nm, I could easily see the beam on a wall in a dark room. I used
my diffraction grating to verify that most of this was continuous spectrum in
the passband of the filter.

The spurious radiation takes the same path that the laser radiation does.

With no filter, I could not see any continuous spectrum with my diffraction
grating. The laser line was so much stronger.

As for IR lasers? If the spectrum is just a long-shifted version of what my
visible laser does, the most visible part of the laser output would be the
laser line. Having a wavelength 100 nm closer to visible increases its
visibility only by about a factor of 1,000 and the total spurious output was
(roughly) 1/1,000 of the laser line output. The wavelength of the bulk of
this was nowhere near 100 nm shorter.

Although I can't be sure this would always be the case, the only spectrum
components I could see using a diffraction grating with my CD player
laser was the laser line at about 800 nm.

I suspect different IR laser diodes may have greatly different ratios of laser
and LED output. If the LED output is only a fraction of a percent of the laser
output, the visible output would be mainly the slightly visible laser line. If
the LED output is equal to a few percent or more of the laser output, then it
may be more visible than the laser line.

Here are a variety of comments on whether light perceived as originating from
near-IR laser diodes - those with wavelengths shorter than about 1,000 nm -
is actually due to the actual lasing line or just the much broader spontaneous
(LED) emission. For some types of laser diodes, it may be a combination.
But various experiments are described below with Ti:Sapphire and dye lasers
that show clear visibility of near-IR wavelengths beyond 800 nm.

The simplest test would be to use a diffraction grating to both view the
spectrum and detect it with a silicon photodiode. If the maximum detected
matches the location of the most visible spot, then you're seening the
lasing line. If the visible spectrum is smeared out or too faint to see
but there is a well defined detected spot, then it's LED emission.

I tested a 780 nm diode laser module in this manner and the results
were quite clear: The IR and visible spots lined up precisely so in
the case of this module at least, what you're seeing IS the IR lasing
line.

(From: Kjell Kraakenes (kkraaken@telepost.no).)

I once used 780 nm laser diodes similar to the types used in CD players, and
something that puzzled me was that I was able to see some red radiation from
these diodes. I used a microscope objective to focus the light on a wall a
few meters away, and when properly focused, a red spot was visible to the
naked eye. I had a piece of black card board on the wall, and there was no
specular reflection. I used an IR viewer of the type sold by Edmund
Scientific (Find-R-Scope), and if I looked at the spot with this IR viewer
the beam appeared defocused. By adjusting the distance between the laser diode
and the microscope objective, the spot (as it appeared through the IR viewer)
could be brought to a better focus. The red, visible light was then so much
defocused that it was no longer visible to the naked eye. From these
observations, I assumed that the spot I saw through the IR viewer was the
laser emission at 780 nm, and that the visible light was some weak emission
at a shorter wavelength. Because of the chromatic aberrations in the
microscope objective these two wavelength could not be expected to be in focus
simultaneously. I did not notice whether the distance between the laser diode
and the microscope objective was increased or decreased when shifting between
the focus of the visible and the IR light, but since I did not know the
chromatic aberrations of the microscope objective this information would not
help me.

I damaged a few of these laser diodes. Probably by burning one of the facets
such that the lasing threshold was increased. Electrically they were OK, and
the visible output appeared as intense as before, but the total output was
only a few microwatts.

I therefore believe that the light people see from NIR laser diodes is
spurious emission within the visible band, and not intense NIR radiation.

(From: Don Klipstein (Don@Misty.com).)

According to the official 'standard observer' photopic response of the human
eye, the long wave cutoff is a gradual one. Sensitivity roughly halves for
each 10 nm further into the infrared. This trend holds close to true enough
'officially' from 700 to at least 780 nm.

It seems as if a small spot is usually (maybe only barely) visible to
dark-adapted eyes in a dark room with eye-safe levels of any wavelength up
to around 880 to 900 nm, maybe 950 nm for brief viewing. (If your eye's long
wave sensitivity is not below average!)

But you may not want to push your luck. A milliwatt of IR can permanently
cook a spot of your retina, maybe within a couple seconds, and with no pain or
warning. Prolonged focusing of any quantity of light over 0.4 microwatt onto a
single point on the retina is potentially damaging, although several
microwatts won't do damage in only seconds.

Be careful if the main beam of the IR laser diode is collimated or not known
to not be collimated. Some IR laser diodes have visible spurious emission,
which may detract you from the main beam. In some other IR laser diodes and
depending on your eyes, most of what you find visible is the main IR
wavelength and you may be exposing your eyes to plenty of it if you find it
visible.

(From: Sam.)

I wonder about this. We use 1 W+ laser diodes at 808, 814, and 980 nm
routinely while monitoring on an optical spectrum analyzer. While we don't
usually search for shorter wavelengths from the diode, we do occasionally
scan for other wavelengths and have never seen any that would explain the red
emission other than the fundamental of the diode. 808 nm and 814 nm are
faintly visible; 980 nm is totally invisible. I have even seen very very faint
red-appearing light from high power 870 nm laser diodes for which the optical
spectrum was known and very local to 870 mn. Thus, it must be that this
wavelength that is actually still visible. Your mileage will vary and
depend on the model and revision level of your set of eyeballs. Consult
factory for more information. Have model and serial number available. :)

(From: Professor Harvey Rutt (h.rutt@ecs.soton.ac.uk).)

I don't know what the dynamic range of your spectrum analyzer is - and
I'm sure the sidebands vary greatly from diode structure to structure.
We have seen large wings on both sides of 780 to 810 nm diodes, sometimes
very structured, sometimes broad and featureless. One 1.48 W diode was
emitting astonishing amounts at 1.9 to 2 um for example. For a 1 W
diode, say 10-9 or -90dB or 1 nW would be easily visible to the
dark adapted eye and if it's in the 600 nm-odd region (where we have seen
emission) it's that you will be seeing not the 1 W of 800-odd nm. The
emission can be very broad, which your eye integrates up but an
analyzer sees as a very flat signal just above noise; remember that
for good dark adaption and narrow electrical bandwidths your eye is not
*that* much worse than a PMT! Incidentally, since the photon has to
cause photochemistry in the eye to get detected, I rather suspect that
the drop in sensitivity with wavelength may well steepen. For example
in my less careful youth I've looked at MW class 1.06 um lasers hitting
things and never seen anything at all unless there is a plasma flash.

(From: Johannes Swartling (j_swartling@hotmail.com).)

I have an external cavity-stabilized diode at 785 nm in the lab, with a
band-pass filter to remove unwanted sidebands. It is clearly visible, and
there is definitely no stray light at shorter wavelengths.

In another lab there's a Ti:Sapphire laser running at 790 nm, and that is also
visible, even when it's running CW (narrow bandwidth).

(From: Harvey.)

Probably the best data I've seen that you can really see it but *certainly* in
many cases it is stray shorter wavelength from diodes, we have measured it. For
1 W class sources a 10-9 level sideband can easily be the cause of
the visibility, especially as the eye integrates up broad band featureless mess
that spec. analyzers easily miss. Its easy to say definitely narrow band. but
what is the bandwidth at the -80, -90dB level? For the Ti:S I guess you
can be pretty sure though - I don't recall how short the fluorescence can go.

However I would still maintain it is very unwise indeed to try. Your eye
sensitivity is down 5, 6 orders of magnitude on peak, it will look dim, but the
potential for eye damage is horrendous - & I'm not a safety 'freak'. Certainly,
to see it, you would have to blow massive holes through laser safety rules!

(From: Josh Halpern (theherd@erols.com).)

What is often missing from these discussions is that there is a fair amount
of variation among people as to how far in the red/blue they can see.
Dye lasers are good tests of this. I can see down to about 380 nm and
also out to about 820 nm. Some people crap out at a little below
400 nm and a little above 780 nm. I know one person who can see down
to 370 nm and well above 840 nm, but he is very unusual.

(From: Roithner Lasertechnik (office@roithner-laser.com).)

2 wavelengths out of one laser diode chip: Yes, it's possible.

Some months ago we receiveed a batch of 980 nm laser diodes (modules) with
light emission at two wavelengths: One as expected at 980 nm (50 mW) and
another very low power emission at around 670 nm (few 10 uW).

You must see it to believe it, but out of one laser diode chip there can be
red light and infrared light, that's fact.

"The spectrum of this laser diode (Sanyo) is supposed to be quite narrow (about
3 or 4 nm) in the range 635 to 645nm. But when I have tested that diode, I
have found that it emits light from 635 nm up to 660 nm!!! So the width of
its spectrum is more than 20 nm!"

(From: Mark Summerfield (m.summerfield@ieee.org).)

Could you give some more details of your measurement?

How did you make the measurement - i.e., with what instrument(s)?

What were the bias conditions of the laser diode (preferably
expressed relative to the threshold current)?

What, exactly, were the results?

These questions should enable us to account for the three most obvious
possibilities:

That your measurement was inaccurate and/or misleading.

That you were not observing lasing at all.

That you do not fully understand what the manufacturer means
when they specify the spectral width.

In each case, the explanation may be:

The measurement must be of sufficiently narrow resolution that when you
observe the power at, say, 660nm, you are not observing significant "leakage"
of light from the main lasing mode at around 635 nm.

If the diode is not biased (sufficiently far) above threshold, you will
see a very broad spectrum including all the cavity modes within the
semiconductor gain bandwidth (typically many tens of nanometers). Only when
the device is lasing will a small number of dominant modes appear.

Spectral width is normally specified as "full-width half maximum", i.e.
the difference (in nanometers) between two points in the spectrum where the
power is one half of the peak power. On a sufficiently sensitive instrument
(e.g., an optical spectrum analyzer with the display set to logarithmic
scale), you will see power over a much wider bandwidth than this. However,
it remains true that "most" of the output power lies within the specified
bandwidth.

One final possibility is that the diode is faulty, damaged, or does not
otherwise meet spec. However, if you are inexperienced in the use and
characterization of laser diodes, we must eliminate all the above
possibilities first.

(From: Harvey Rutt (h.rutt@ecs.soton.ac.uk).)

Most laser diodes emit a broad background of spontaneous emission as well as
the laser output.

A student of mine made another error a while back. He simply had the gain on
the detection system turned up too high; the very narrow laser line was
heavily saturating the system, and he saw those big broad wings.

Which incidentally can extend extraordinary distances and have all sorts of
structure. One of our 810nm diodes puts out a load of broad band mess out near
2,000 nm (yes, 2 um!) but virtually nothing in the 1 to 1.8 um region.

The dominant wavelength is the wavelength (mixed with white if necessary) that
matches the color of the light source in question. The white, if not
specified, is usually C.I.E. Standard Illuminant C which is approx. 6500
Kelvin. C.I.E. Illuminant E, which has chromaticity of (.3333, .3333) and is
very slightly purpler than approx. 5500 Kelvin, may also be used. Most LEDs
are either close enough to matching a spectral color or on a blue-yellow line
that most whites are close to that it is not really necessary to specify the
white.

But here are the peak wavelengths, dominant wavelengths, and approximate
limunous efficacies (lumens in each watt out, not lumens per watt in that
I mention in The Brightest and
Most Efficient LEDs and Where to Get Them! for various LEDs. The luminous
efficacy of 555 nm is approx. 681 lumens per watt.

Please note that I have misplaced some Hewlett Packard LED datasheets
which contain most of the luminous efficacy data that I had on hand. I
may be able to recover some from Hewlett Packard's web sites and refine
this later.